I have a .psd file that is getting this error message "Could not complete your request because it is not a valid Photoshop document."
This is the first time this has happened on my Ps, CS6...and I've looked elsewhere to get some help.

And I was just working on this file a while ago, so I'm not sure what went wrong.
It is located on one of my USB Flash Drives, and I have heard that certain USBs tend to cause these things....yet this is the first its happened

Sounds like the file is corrupted. you should never, EVER "work" on any document, file or whatever "from" the USB. This is likely going to cause the issue you are having. The file is probably not recoverable as it IS there but corrupt. There may be a 3rd party .PSD file repair/fix/get-what-you-can typeutility about, but you'll have to search. If you dont have a backup on onother drive, not sure what else you can do unfortunately

maybe you have an auto backup ?!

Clone the flash drive, then search the clone for a deleted copy of the desired file.

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A backup a day keeps DR away.