I am running Shibby Tomato on a Linksys E2500 v3. I have a 1 TB external HDD which is formatted as a single ext3 volume. I tried using it with the router as follows:
I went to to USB and NAS > USB Support, and checked USB Storage Support, and "Ext2 / Ext3" (Automount was already checked)
Pressed "Save" and plugged in the hard drive. It doesn't show up under "Attached Devices". I also didn't see anything in /mnt (or /tmp/mnt) when I SSHed into the router. I noticed that there was a message "Some of the changes will take effect only after a restart.", so I rebooted the router. After it was finished rebooting, I went to the USB Support page again, and "USB Storage Support" was unchecked.
I'm a complete novice to Tomato (Just got a Linksys after having a Belkin router previously), and I've had other issues where I thought I was making persistent changes, only to see them reverted when I went back to the same page, so it's possible I'm doing something wrong.