Don't ever save directly to any kind of removable drive. Always save to the HD, then copy to the flash drive. Similarly, don't open a file from the flash drive and work on it. Copy to the HD first, then work on it from there. At the very least, if you must
work on a file from a flash drive, go to File | Options | Advanced | Save and check the option to "Copy remotely stored files onto your computer, and update the remote file when saving."
The reason for this is that Word creates many temporary files in the same folder with your document. Although the problem is less acute with a flash drive, it was especially dangerous to work from a floppy because Word could very quickly fill the floppy
with temp files. Also, every time you save, Word does a lot of file swapping to replace the current file with the new version and to save the old one as a backup (see
“Description of how Word creates temporary files”). If you remove the drive before Word is through with this operation, then you will
have hopelessly corrupted the file.