New development, when I inserted the USB thumb drive again, the files were there, so they didn't get wiped out.
Thought I'd try "Eject..." from windows prior to removing it, then inserted it again to the PC. No errors and it's writable again!
Also, while on the subject, try not to remove the micro SD card from your Pi while it's running.
Well, after shutting down the Pi first, then removing a Sandisk Cruzer thumb drive, same problem. Shutting down the Pi before removing the d rive doesn't seem to improve things.
Plug it into a PC, and get
"There might be a problem with some files on this device or disc. This can happen if you remove the device or disc before all files have been written to it."
Clck Scan and Fix, and Windows finds no problems.
Files are readable, but can't write to it, at first, but now I can - very strange! This is only a 65 gb drive, but I'm real concerned about a 500 GB SanDisk SSD and using it in the Pi. I couldn't fix that problem before, and I have 168 Gb of files on it (so far). Don't really want to risk not being able to add more music to it, so I guess I need to continue filling it up before using it w/ the Pi !
Off to check SanDisk website on this problem.