I thought the original issue was you could read but couldn't write
to the troublesome thumb drives.

My original reply in that regards was to check "ownership" of the thumb
drive, and I don't recall any reply.

Things to check are the filesystem type and the drive ownership. On the
assumption the thumb drives do automount:

1. From the command line in a terminal window:

a) df -T

reveals the filesystem type in the second column
mine show as "vfat" in Ubuntu 8.04, "FAT" (not "FAT32") in Vista

b) ls -ld /media/drivename

reveals ownership in 3rd column, group in 4th column; e.g.,
drwx------ 9 thad root ...

2. from the desktop GUI, right click the thumb drive's icon, select
"Properties". On the "[Basic] tab, mine is "Filesystem type: msdos".
On the [Volume] tab, mine shows "Filesystem: vfat (FAT16)", and
several lines down shows "File System: vfat"/"rw nosuid ...".
The [Permissions] tab claims permissions cannot be obtained, though I
can read and write from/to the thumb drive(s) just fine.

If your thumb drives cause different displays from the above, that
will be a clue to the nature of the problem.

FWIW, my thumb drives are 2GB and 4GB SanDisk.