O.K. Thanks, I'll check that out.

O.K. Thanks

o.k. That's what I was afraid of. So , how can that be avoided in the future ? If I can go back and find the computer they are in, and put them back on my USB, how can I keep that from happening...

Sorry, I don't know about this stuff. A couple of years ago someone downloaded these ESL teaching books for me from his USB to mine.
Then someone else downloaded them onto a class computer I was...

I'm an ESL teacher. Some months ago I had someone download some books I had on a USB stick onto a computer I using at that time. Yesterday I tried to download the books onto another computer and was...