The Seagate FreeAgent external USB drives and other large external USB drives, when formatted as EXT3?, will not be mounted if they have an inode size of 256. You can check the inode size of a drive by mounting it on another Linux computer and running as root:
dumpe2fs -h /dev/sda1|grep Inode
Currently, the only solution is to re-format the drive with an inode size of 128 or format the drive as NTFS.
mke2fs -v -I 128 -j /dev/sdX1