Windows cannot open or copy a .DAT file (originally a .DOCX) after reinstalling Windows – shows encryption/permission error

I had a normal .DOCX file in my previous Windows installation. A long time ago, I manually changed its extension to .DAT (I did not use encryption or EFS).

After reinstalling Windows (clean install), Windows now refuses to:

• open the file
• copy it
• rename it
• or open it even after changing extension back to .DOCX

Windows shows a message related to encryption or permission, as if the file is locked or encrypted.

Important notes:

• I never enabled “Encrypt contents to secure data”
• The file was inside my previous Windows user profile
• I no longer have access to the old Windows user or the old SID
• Even with antivirus (NOD32) disabled, the behavior is the same
• Other files copied from the old Windows work fine — only this .DAT file is locked

It looks like Windows thinks the file belongs to the previous user SID or considers it encrypted, but I cannot decrypt or take ownership.

How can I regain access to this file? Is there any method to override the encryption/permissions or recover the file contents?

Most .DAT files are binary files, sometimes they are just plain text files.
You can't just rename a .DOCX file, which in fact is a XML format, and expect it to function as .DAT.

You could, if the desired destination file has to be a plain text file, write that file into plain text (within Word or similar app).
And then rename it to .DAT. That should function.

I am not surprized that MS has decided to avoid the use of renamed .DOCX files as .DAT. files.
Would have expected they would do that earlier already.

BTW. I did a search for .DAT files on my system drive, and changed the extension of 5 of them (after copying them to a scratch area).
The files that I took were:
1. aeheur_gwf.dat
2. amde31a.dat
3. EdgeWebView.dat
4. tsprint-datafile.dat
5. wia.dat

Then I changed the extension to .txt and tried to open them.
1 to 3 reported to be binary files
For instance file 1 was looking like this:



While file 4 showed pure text:

This is the data file for the Terminal Services Easy Print
printer driver. It does not contain any real data - it is just
that the printer driver model requires us to have a data file,
and we oblige!

Sincerely,
G.a.u.r.a.v. D.a.g.a
Z.a.r.d.o.s.h.t K.a.s.h.e.f.f
V.i.s.h.a.l M.a.r.u
M.a.k.a.r.a.n.d P.a.t.w.a.r.d.h.a.n
P.r.i.y.a R.a.g.h.a.v.a.n
V.l.a.d.i.m.i.r S.t.o.y.a.n.o.v

Click to expand...

And that happens with many files.

I will try to simulate your problem and see if I can reproduce it, by renaming a .DOCX file into .DAT., but that takes time.

I did try 3 files, that were normal .docx documents. Changed them to .dat files and opened them in the notebook app.
I got the warning that they were binary files, nevertheless opened them and they looked like this:




But when I renamed them into .docx, I could use them again in a similar app like Word!

So I have not been able to reproduce your problem.
Or did you try to use that .DAT file in some program, different from the notebook app I used?

You might be able to unlock the file by using an unlocker program.
I use the freeware program ThisIsMyFile for that.
Download adress: ThisIsMyFile 4.44 Download
But there are more of such programs!

Please show a screenshot of the actual error. There may be detail there that could help us help you.

Check your Windows Defender settings and ensure that Controlled folder access is OFF.

Then try this simple test:

• Create a text file (i.e. right-click desktop and select New > Text document)
• Change the text file's extension from .TXT to .DAT
• Double-click the file. Do you get the same error?
• Right-click the file and select Open with. What is displayed (post a screenshot)?

mehdi1976 said:

Windows now refuses to:
...
open it even after changing extension back to .DOCX

Click to expand...

1 Please post a screenshot of the error dialog shown when you try to open it as a .docx file.
[Do please crop the rest of the screenshot out so you are only posting the bit that matters.]

2 Which applications have you opened the file in since renaming it as .dat?

3 How does its Date modified compare to the date you last opened it in MSWord?

4 What were you trying to achieve by renaming it to .dat?


Denis

Use this free online converter to Convert your .DAT file to .DOCx