I used E for my usb drive, oracle and db2. Now it is G. and E is not on the list anymore.
If you plugged it in to a different USB Port than the usual one, XP will be confused and think it's a new device. E has already been reserved for what XP thinks might still turn up one day so it allocated a new letter. In the drive's Properties you can allocate a fixed letter from the unreserved list if that helps.