Excel should be closed or at the very least the file should be closed when you copy.
You can't count on being able to recover a missing file, and that is the reason
that having backups is important.
I find the XCOPY command to be very reliable for copying files.
The thumb drive is essentially a disk drive itself, so I would run a utility on it first,
and then on the hard drive to see if you can get back the missing file.
Disk Utilities - backup, recovery, status (#refUtil)
http://www.mvps.org/dmcritchie/excel/backup.htm#refUtil
You are trying to locate fragments of file for which there is no longer a directory
item, you may have success or failure, but its about as reliable as losing a $10
bill on a city street and seeing if you can find it again after a few hours. Some
cities are known to be windy. Good Luck. I've managed to recover files
that I needed. But I expect if I "really, really, needed the file" that I probably
wouldn't be as lucky. In any case it is best to already have the utilities you
need already installed so you don't overwrite what you need, and to run them
as soon as possible..