I had some data backed up onto an external hard drive, and it seemed fine. When I reopened the folder a few minutes later, there were only two files left instead of hundreds.  I can't think of anything  I was doing that might have caused this. I wasn't formatting or deleting anything, I wasn't doing anything to file systems, I wasn't doing anything that might have caused this.  The two files left are the two most recently edited.   find and locate commands don't show them anywhere else. Well, sorta: /home/user/.local/share/Trash/files/ contains all the directories, but they're all empty.  Is it hopeless? Is there anything I can do?
About two weeks ago I had a problem with my PC that my brother wrote a question about at .  I still haven't solved the problem, so my PC is unusable now.  Now I think that it would be simpler to re-install both Win7 &amp; Ubuntu than repair them, but there're some data on their partitions. I want to recover this data.  These partitions are not mountable for some reason. I don't know why.  sudo fdisk -l about the problematic partitions:     Device    Boot      Start         End      Blocks   Id  System /dev/sda1            2048     147912703    73955328   83 Linux /dev/sdb2            206848   174079999    86936576    7  HPFS/NTFS/exFAT   And in GParted:      And the error of the mount command:  Failed to mount '/dev/sdb2': Invalid argument The device '/dev/sdb2' doesn't seem to have a valid NTFS. Maybe the wrong device is used? Or the whole disk instead of a partition (e.g. /dev/sda, not /dev/sda1)? Or the other way around?   And the exact same message for /dev/sda1  How can I recover my data from those HDDs?  EDIT     I have tried to specify the type of the drive:  $ sudo mount -t ext4 /dev/sda1 /olddisk mount: wrong fs type, bad option, bad superblock on /dev/sda1,        missing codepage or helper program, or other error        In some cases useful info is found in syslog - try        dmesg | tail  or so $ sudo mount -t ntfs /dev/sdb2 /olddisk NTFS signature is missing. Failed to mount '/dev/sdb2': Invalid argument The device '/dev/sdb2' doesn't seem to have a valid NTFS. Maybe the wrong device is used? Or the whole disk instead of a partition (e.g. /dev/sda, not /dev/sda1)? Or the other way around?   EDIT     sudo ntfsfix /dev/sdb2 returns:  Mounting volume... NTFS signature is missing. FAILED Attempting to correct errors... NTFS signature is missing. FAILED Failed to startup volume: Invalid argument NTFS signature is missing. Trying the alternate boot sector Unrecoverable error Volume is corrupt. You should run chkdsk.
I was asked today in class why you divide the sum of square error by $n-1$ instead of with $n$, when calculating the standard deviation.  I said I am not going to answer it in class (since I didn't wanna go into unbiased estimators), but later I wondered - is there an intuitive explanation for this?! I'm seeking for a proof of the following:     Let $A$ be an invertible matrix. Then the determinant of $A^{-1}$ equals:   $$\left|A^{-1}\right|=|A|^{-1} $$   I don't know where to begin the proof. Any suggestions? What is the C# Using block and why should I use it? What is the purpose of the Using block in C#? How is it different from a local variable? \colorbox height too long \documentclass{article} \usepackage[utf8]{inputenc} \usepackage{xcolor} \usepackage{minted} \usepackage{parskip}  \definecolor{codegray}{gray}{0.9} \newcommand{\code}[1]{\colorbox{codegray}{\texttt{#1}}}  \begin{document}  \code{test code}\\ anotherline  \end{document}   This is working good on sharelatex.com and looks like I expect and want it:   but on my local project using this command gives me this:   any idea what could be causing this colorbox height going to the next line? Short, Politically Correct word for Native Americans No more than four syllables, more PC than Indians.  EDIT: I arbitrarily chose four syllables because any more seemed like a mouthful. I like to be PC and not have to stumble over 6+ syllables. For example I want to use \iint from ams package  and also mathabx package in one file together. Actually I want a command that neutralizes ams package effect and replace it with mathabx package effect from here on in my file. Thanks for your responsibility.  This is my example minimal file:  \documentclass[12pt,a4paper]{report} \usepackage{amsmath}  \usepackage{mathabx}  \begin{document} i want this one in ams style:        \[ \iiint\,\iiint\, \iiint\,\oint\, \] text text text text text text text text text text text text   and this one in mathabx style:   \[ \iiint\,\iiint\, \iiint\,\oint\, \]   but both of them are the same and i don't like it ..... \end{document} Moment of inertia: why $\mathrm{dI}=r^2\mathrm{dm}$ instead of $\mathrm{dI}=m\mathrm{dr^2}$? public static IQueryable&lt;TResult&gt; ApplySortFilter&lt;T, TResult&gt;(this IQueryable&lt;T&gt; query, string columnName)   where T : EntityObject {   var param = Expression.Parameter(typeof(T), "o");   var body = Expression.PropertyOrField(param,columnName);    var sortExpression = Expression.Lambda(body, param);   return query.OrderBy(sortExpression); }   Because the type for OrderBy is not inferred from sortExpression I need to specify it something like this at run time:  var sortExpression = Expression.Lambda&lt;T, TSortColumn&gt;(body, param);   Or  return query.OrderBy&lt;T, TSortColumn&gt;(sortExpression);   I don't think this is possible however as TSortColumn can only be determined during runtime.  Is there a way around this? I've start a new job today and I am trying to set up my machine to run through their Windows server.  I've managed to get a internet connection through the server now but now I can't run apt-get update as I get a "403 Forbidden" error.  This is for every repo under my source list, apart from translations(?).  I do have a proxy in apt.conf, if I don't have it I get a 407 Permission Denied error.  Here's my apt.conf file (I have omitted my username and password)  Acquire::http::proxy "http://username:password@192.168.0.2:8080/";   Here's my sources.list  #deb cdrom:[Ubuntu 12.04.2 LTS _Precise Pangolin_ - Release amd64 (20130213)]/ dists/precise/main/binary-i386/  #deb cdrom:[Ubuntu 12.04.2 LTS _Precise Pangolin_ - Release amd64 (20130213)]/ dists/precise/restricted/binary-i386/ #deb cdrom:[Ubuntu 12.04.2 LTS _Precise Pangolin_ - Release amd64 (20130213)]/ precise main restricted  # See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to # newer versions of the distribution. deb http://gb.archive.ubuntu.com/ubuntu/ precise main restricted deb-src http://gb.archive.ubuntu.com/ubuntu/ precise main restricted  ## Major bug fix updates produced after the final release of the ## distribution. deb http://gb.archive.ubuntu.com/ubuntu/ precise-updates main restricted deb-src http://gb.archive.ubuntu.com/ubuntu/ precise-updates main restricted  ## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu ## team. Also, please note that software in universe WILL NOT receive any ## review or updates from the Ubuntu security team. deb http://gb.archive.ubuntu.com/ubuntu/ precise universe deb-src http://gb.archive.ubuntu.com/ubuntu/ precise universe deb http://gb.archive.ubuntu.com/ubuntu/ precise-updates universe deb-src http://gb.archive.ubuntu.com/ubuntu/ precise-updates universe  ## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu  ## team, and may not be under a free licence. Please satisfy yourself as to  ## your rights to use the software. Also, please note that software in  ## multiverse WILL NOT receive any review or updates from the Ubuntu ## security team. deb http://gb.archive.ubuntu.com/ubuntu/ precise multiverse deb-src http://gb.archive.ubuntu.com/ubuntu/ precise multiverse deb http://gb.archive.ubuntu.com/ubuntu/ precise-updates multiverse deb-src http://gb.archive.ubuntu.com/ubuntu/ precise-updates multiverse  ## N.B. software from this repository may not have been tested as ## extensively as that contained in the main release, although it includes ## newer versions of some applications which may provide useful features. ## Also, please note that software in backports WILL NOT receive any review ## or updates from the Ubuntu security team. deb http://gb.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse deb-src http://gb.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse  deb http://security.ubuntu.com/ubuntu precise-security main restricted deb-src http://security.ubuntu.com/ubuntu precise-security main restricted deb http://security.ubuntu.com/ubuntu precise-security universe deb-src http://security.ubuntu.com/ubuntu precise-security universe deb http://security.ubuntu.com/ubuntu precise-security multiverse deb-src http://security.ubuntu.com/ubuntu precise-security multiverse  ## Uncomment the following two lines to add software from Canonical's ## 'partner' repository. ## This software is not part of Ubuntu, but is offered by Canonical and the ## respective vendors as a service to Ubuntu users. # deb http://archive.canonical.com/ubuntu precise partner # deb-src http://archive.canonical.com/ubuntu precise partner  ## This software is not part of Ubuntu, but is offered by third-party ## developers who want to ship their latest software. deb http://extras.ubuntu.com/ubuntu precise main deb-src http://extras.ubuntu.com/ubuntu precise main   I can sort-of fix this by changing all the http in sources.list to ftp but I still have issues with ppas QGIS Align Rasters how to retain pixel values? I am attempting to align and then merge a number of rasters of the same resolution and extent, but where the pixels are slightly misaligned (creating small gaps when merged). This works fine using the Align Rasters tool, except that the pixel values are not retained and instead given a value of something like -3.23879e+8. I mostly want to keep the nodata pixels. Is this possible? I am currently in hard mode and during a blood moon, several of my NPCs including my guide and my healer died. it's been several in game "days" now and they have yet to respawn. Is there a way to check why they aren't respawning?   Is there a list of reasons for the NPCs to not respawn?   Edit in reponse to lunbok's answer:  1. They don't have appropriate housing - There is an in-game housing query function (changes your cursor into a ?) that lets you click on your house to see if it is still a suitable house and I clicked in every room in my house and they are all listed as "This housing is suitable" or "This housing is already occupied".  2. Their spawn conditions aren't met anymore. - This doesn't explain why my guide isn't respawning since they don't need a condition to respawn (other than having a suitable housing which the point above already prove isn't the problem).  3. There's a goblin invasion going on. - Do the invasions last that long? It has been about two reals days and a lot more in-game "days" now. As far as I can tell there shouldn't be an invasion going on.  4. It could be because you built your house in a part of the world that is now corrupted Can you use Magic Initiate to cast a spell you already know, and still learn another spell? I have a wizard PC who wants to gain the feat Magic Initiate and use free 1st level spell to use mage armour once a day. He already knows Mage armour, so:   Can he use the Feat to gain a free casting of Mage armour once each day? Should he get another 1st level spell but not get to freely cast it once a day? Report this Ad Feature Consider the closed interval $[0,1]$, there is $\frac{2}{3} \in [0,1]$ where $p=2$ and $q=3$. Similarly consider $[2,3]$, one can have $\frac{5}{2} \in [2,3]$ where $p=5$ and $q=2$. Does every interval of the form $[a,b]$, where $a,b \in \mathbb{R}$ contain a rational of this kind. If yes, how can we prove it? Quoted vs unquoted string expansion Propagate all arguments in a bash shell script How to prove the midpoint of a chord is also the midpoint of the line segment defined by the points of intersection of other two chords with it? Bernhard Elsner, alias MathOMan, posted this exercise in plane Geometry,  on January $29$th, $2010.$  "Let $\mathcal{C}$ be a circle, $A,B$ two distinct points on $\mathcal{C}$ and $M$ be the midpoint of the chord $[AB]$. Take two other chords,$[PQ]$ and $[SR]$, that pass through $M$. Let $C$ (resp. $D$) be the intersection of $[AB]$ with $[PS]$ (resp. $[RQ]$). Prove that $M$ is the midpoint of the chord $[CD]$."    To prove it I've written the following (failed) argument, in the  of this post (translation of mine):    The figure is symmetric with respect to $M$: $\overline{AM}=\overline{MB}$, $\overline{PM}=\overline{MU}$, $\overline{RM}=\overline{MW}$, $\overline{QM}=\overline{MV}$, $\overline{QR}=\overline{VW}$, $\overline{SM}=\overline{MT}$. From $\dfrac{\overline{SC}}{\overline{DT}}=\dfrac{\overline{CM}}{\overline{MD}}=1$ follows that $\overline{CM}=\overline{MD}$.  Here is an extract of the author's reply (translation of mine):   "It is not clear that  $\overline{QM}=\overline{MV}$. Is the point $V$ on the line $(QM)$ defined by this equality or is $V$ defined as the intersection point of the lines $(QM)$ and $(WC)$? Why are both definitions to give the same point?  Let $C^{\prime }$ be the intersection of $(SP)$ and $(VW)$, and $D^{\prime }$ the intersection of $(TU)$ and $(QR)$. (...)  One still has to show that $C^{\prime }=C$ and $D^{\prime }=D$."   I have agreed with these objections.  Until now no proof has been posted. The author considers that the "proof is not quite simple".  Q. What is the theorem this exercise refers to? Or how does one prove it? I am trying to solve the following exercise:     Prove that complex multiplication does not extend to a multiplication   on $\mathbb R^3$ so as to make $\mathbb R^3$ into a real division   algebra.   I am aware of Frobenius' theorem that there are only three finite dimensional real associative division algebras. But the exercise would be pointless if the theorem was assumed.   Let $x,y \in \mathbb R^3$. Since $x y$ has to extend the complex multiplication:  $$xy = (x_1y_1-x_2y_2, x_1y_2 + x_2 y_1, ?)$$  I have no idea how I can proceed from here. Could someone tell me how to do this? It should be easy as the other exercises I did so far were also easy. Host alias for ssh There are a few SSH destinations I will frequently be connecting to and I'm wondering if rather than remember the IP address I can create a host alias for them.  I'd like to be able to use...  ssh -p xx user@domain.ssh  I've tried establishing this host in /etc/hosts and unless there's a service I needed to restart it had no effect. Answering beyond the question's scope Is answering beyond the scope of the question a valid thing? Or should we just stick to the question asked and ignore any potential bugs which might be present (of course OP is unaware of that, as of now).  To make myself more clear, please have a look at this .  There are 3 answers which talk about the issue asked and that's it. If you look at my answer, I've added an additional point about a potential bug which the OP has missed now (may find out later). So is it a good thing to point out such potential bugs or not? Should this practice be encouraged?