I recently had a problem with one corrupt file on my 512 MB thumb
drive. In attempting to make this file readable, I somehow converted
every file on the drive to a nonreadable 4K file plus a folder called
FOUND which contains 9999 FILEnnnn.CHK files.  How can I convert these
FILEnnnn.CHK files back into program files and restore my thumb drive?

I would suggest that if these files are only worth $2.00 to you, you
might as well type them back in yourself.

Recovering data from FILEnnnn.CHK files is dicey at best. These files
represent sector dumps of the drive. You can't accurately convert them
back into files without knowing the contents of the drive FAT before
the problem happened. If you can identify which file each FILEnnn.CHK
piece belonged to (and you can work out the correct order), you MIGHT
be able to recover a partially usable copy by gluing together the
FILEnnnn.CHK pieces in the right order (you can do this with a command
line like copy /b file0001.chk+file0002.chk+file0003.chk >
my_file.doc) but you'd still have to identify the correct end of the
file (the last FILEnnnn.CHK piece will have an undetermined amount of
garbage in it).

Fundamentally, if you have plain text files you have a chance to
recover them from FILEnnnn.CHK pieces. Anything significantly non-text
and you're more or less sunk. With Word .docs you might be able to
recover a reasonable portion of the text, if you're lucky, but you'll
almost certainly lose all your formatting. Don't even bother with
Excel .xls spreadsheets, you'll get nowhere.