I use a USB thumb drive (a Toshiba 2GB with the U3 software uninstalled) for many things; my SSH and GNUPG files along with the pet software projects I'm working on.

Today, I inserted it into my laptop (running Ubuntu 9.04 Jaunty Jackal) and it didn't automount. I suspected file system corruption on the thumb drive, & I was right. The fix was easy:

sudo apt-get install partionmanager
sudo partitionmanager

I selected the thumb drive from the partition list, right-clicked, and chose Check. Then I clicked Apply, and partionmanager repaired whatever faults existed on the filesystem. The thumb drive then auto-mounted with no further effort.

Oddly, my first attempt to rectify this from the command line using fsck failed. The file system check completed, but the drive still wouldn't mount. I'm guessing there is a difference between what fsck did and what partitionmanager did during repair, but I don't know what it is ...