How To'sWindows

Fix: Invalid MS-DOS Function Error

Invalid MS-DOS Function comes up when you try to move, delete, copy or rename files. This is a file-system error which usually affects the file functionality only, but since it is very annoying it limits your usage. The error can come up on Windows 10 and it’s predecessors (Windows 7, Vista, 8 and 8.1). Methods described in this guide will work on all versions of Windows (stated above). Due to numerous causes of this error, i have not found a single fix that works; for many users a mix and match of fixes has worked which i have listed in this guide. It would be best to go with each method described below and test after each method until it is fixed.


Optimize and Defragment Your Drive

Hold the Windows Key and Press E. Choose This PC from the left pane if you are using Windows 10/8 or 8.1. If you are using Windows Vista /  7 then proceed to steps below.

Right-click your Disk Drive and choose Properties. Then click the Tools Tab and choose Optimize (Windows 8/8.1/10) and Defragment (Windows Vista/7) and run it.

Invalid MS-DOS Function-3

After the disk has been defragemented; reboot your system and test.

NTFS Files To FAT32 Format

If you are copying files from a NTFS format to FAT32 Format, then  run the hotfix below.

Edit System Policy via Registry

Hold the Windows key and Press R; then type regedit and click OK


Navigate to the following path in the registry editor


Then, in the right pane ; right click and choose NEW and select DWORD Value

Invalid MS-DOS Function-1

In the value name field; type CopyFileBufferedSynchronousIo then press ENTER.

Right click the newly created value CopyFileBufferedSynchronousIo, and choose Modify

Type 1 in the value data box and hit enter/ok. Then reboot your PC and test to see if it works. If it still won’t work then process to Method 3.

Invalid MS-DOS Function-2

Once the above has been modified, go to Command Prompt; and copy the data to hard disk using the copy commands.

Run Chkdsk to CHECK DISK for Errors

Instructions for doing a chkdsk can be found at:


  1. This is a very good summary of the different fixes. I am trying to copy from a UDF file system (DVD) to an NTFS file system (external hard drive) and getting the “Invalid MS-DOS Function ” error. Optimizing and defragmenting and Chkdsk are not going to work on the DVD, so the only relevant one is the regedit, which I have done and the problem is still there. WIN 7 BTW. Any other ideas?

  2. Do they open if you open them directly? If they are able to open, can you try attaching them to your e-mail and forwarding to yourself? This will rule out corruption possibility. I’d suggest checking folder permissions as well.

  3. Good thought. Thanks for that. No, the files will not open directly from the disk, which would suggest corruption of the files. The odd part about that is, I made two copies of the DVD-ROM at the time as backup disks, and they are both failing in exactly the same way, exactly the same files. Again, there must be a clue in that. They are double layer 8G DVDs, if that matters.

  4. That is a good thought, too. Unfortunately, the souce files were deleted when I made the ‘backup’ disks. There is no particular reason why these same files should be corrupted on both back up disks, and not any others. Since they are double layer, 8G dvds, I am guessing that these files are on the second layer, for whatever reason, and the drive is unable to read them because of that. Again, I am guessing and am not aware of a fix. Also I am not sure why Win would not copy a corrupted file. Is that in the OS?

  5. You made back up from a single source; whether you do it to 1 bd, or 3 – the source stays corrupt. If they were recently deleted, try recuva (a free program) to do the recovery and see if the source opens.

  6. The story now gets even more bizarre. I came in this morning, put the DVD in the drive and attempted to copy again – and they all worked?! All the files that were giving me the “Invalid” error yesterday copied from the UDF DVD ROM to the NTFS USB external hard drive via Win drag and drop. I guess the problem is solved, but I have no idea why. I did nothing but run Recuva on the computer internal hard drive to see if any of those files were recoverable on it. None were. I can only think that the DVD drive is a bit ‘touchy’ about reading those double layer DVDs. Taking it out and putting it back worked, although I did try that before without success. Thanks for your comments. I made no sense that those files were corrupted and none of the others on that disk.

  7. Thank you for the comment. I will shortly update the post with the suggestion you’ve provided that worked. Wish i’d gotten the privilege earlier to suggest (taking it out and putting it back in). Power on/off, in/out, restart should always be the first approach.

  8. Yes. I put it in a Dell laptop with a 1/2 height dvd drive, which I would not expect to be one of the best readers, and it works perfectly. I pt the same disk in the tower and it gives the sale “Invalid” error. That would suggest maybe that drive is the problem.

  9. It is interesting that the laptop does not have that entry in the Registry. It does copy from the disk to the hard drive with the DOS copy command.

  10. In the picture, you had CopyFileBufferedSynchronousIO but in the text you had CopyFileBufferedSynchronousIo. Is it case sensitive?

  11. Thank you. The registry edit worked a treat. I can now manipulate these previously problem files perfectly.

    I entered CopyFileBufferedSynchronousIo not CopyFileBufferedSynchronousIO

  12. Hi Kevin,
    Like others, changing the registry worked a treat. Thank you SO MUCH. This issue has been slowly driving me mad.
    However, I am worried that I could not follow the next instruction “Once the above has been modified, go to Command Prompt; and copy the data to hard disk using the copy commands”.
    I am not an IT technician and cannot figure out how to do this without step by step instructions.
    Also, as I have not done this part of the procedure, is something else now likely to go wrong with my computer as a result?
    Thank you, Paul

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Fix: Invalid MS-DOS Function Error

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