Helping people with computers... one answer at a time.

Looking for large files when running out of disk space makes sense. We look at the steps to determine whether a file, such as a .dat file, is important.

My C: and D: drives have many gigabytes of .DAT files under "Documents and Settings". I'm trying to free up space on the C: drive. Can I delete any of these files? I really don't know what they are, how they got there, or why they occupy so much space on my computer!

I don't know.

Honestly, I have no idea either as to what they are, how they got there, or why they occupy so much space. That's the problem with ".dat" files - there's no way to know what they are without more information.

But I do have some ideas on how to determine if deleting them is ok, and ways to do it safely. And those ideas apply to any file type, not just ".dat".

I'll reiterate a very common response: there's no way to know what a ".dat" file is without knowing the application that created it. The file extension ".dat" is often used to represent "data", and as a result is used by many, many applications. There's no way to know what the file contains, or what to do with it without which application created it.

So, how do you determine if a file - any file - matters?

Well, my first suggestion is perhaps the most important one:

Before you do anything, backup the files.

Copy them to CDs, copy them to another computer, just do something, anything, such that if you discover that you really wanted that file after all, you can get it back. You may find that the file was an important part of some application that you rely on daily. Or worse, an application that you only use once a month or so. If you simply delete the file, it's gone without reasonable hope of recovery. If you've backed it up somehow, you'll not only be able to remove from your system, but you'll be able to put it back if you discover that was a mistake.

" something, anything, such that if you discover that you really wanted that file after all, you can get it back."

My next steps are fairly simple:

First, rename the file or files. Use your computer for a while. Reboot. If an error occurs related to the now missing original file name, you now know what the file was about, and can decide appropriately based on the error you get. If you discover no errors, then you're ready to move on to the next step.

Next, delete file or files. (You did back them up, right? Right?). Same exercise ... use your computer for a while. Reboot. Use it for different things, exercise some of the applications you have on the machine. Once again, if an error occurs related to the now missing file, you now know what the file was about, and can decide appropriately based on the error you get. If you discover no errors, then you're done. Sort of.

Last, remember that you deleted the file and where you put the backup. This is probably the hardest part for me :-). The reality is that our simple tests above may not actually have fired up the application or scenario that needed the file. You may not find out until some lengthy time down the road that ... whoops! ... the file you deleted some months ago turned out to be part of an application you haven't touched in ages. Hence, again, the importance of the backup.

What if I can't delete or rename the file? If it's because the file's in use, that's great! Why? It tells you that the file matters, and you can then use a tool such as Process Explorer to figure out who's using it. Once you figure that out, you can once again decide the right course of action based on knowing what application is accessing the file.

Article C2841 - November 16, 2006 « »

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Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

Not what you needed?

February 8, 2007 5:46 PM

Is there a site or software that can test the .exe files to determine if it is safe to install?

Leo Notenboom
February 9, 2007 4:20 PM

Hash: SHA1

I believe this article might be a good place to start:

Version: GnuPG v1.4.6 (MingW32)


May 16, 2008 8:41 AM

Is it safe to delete every single .log file on my computer? I noticed it takes up a lot of space, and quite honestly I do not want logs of everything going on in my computer. So, is it safe to do or will it hurt my computer somehow?

May 16, 2008 11:25 AM

Hash: SHA1

If you're not sure, just follow the instructions in the
article. :-)


Version: GnuPG v1.4.7 (MingW32)


July 28, 2008 2:08 AM

I already have .dat files or .txt files but they never be more than 50 MB , Surely I'm too still didn't know is these all files necessary or not but generally it's not harm for me to let them without deletion , thanks Leo for useful article ..

October 10, 2008 10:49 AM

I received a low storage message which sugggested deleting unnecessary files. My hard drive is a 30+GB and thought it unusual to be low so I check and sure enough, I'm out of room. Question is can I safely move files from the C drive to a secondary hard drive, which files are ok to move, and how do I go about moving them to free room on my C drive?

I would first try to figure out what's using up all that disk space. This article would be a start: Where's my disk space going?
- Leo

November 17, 2009 9:00 AM

Jhon, instead of deleting the files yourself you should use a maintenance program like CCleaner or Advanced System Care - both are free , safe and easy to use.

Bill Chubb
November 17, 2009 9:15 AM

I'm not suggesting this is a particularly sophisticated solution but if you make a diary note of the date and delete files to the Recycle bin and then, umpteen weeks or months later, if you find you've accidentally deleted something you need, you can go to the Bin and restore it.

Ask-Leo Rocks!

Tom Bassett
November 17, 2009 10:30 AM

If you do a search using "*.dat", you get a lot of info on just what the .dat file is related to, plus a date and size of the .dat file. Doing this you find files for programs you have removed from the harddrive, or are real OLD.

Fran Perry
August 31, 2010 7:30 PM

I wanted to know if you get rid of the "back up copies" of your system .dat and the user.dat files?....I am not interested in backing up the files that have already been backed up...I would appreciate the answer to this question..thanks
Fran Perry Windows XP

Not sure which backups you're referring to. (But in general: better too many than too few.)

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