I followed the instructions at http://www.bumwine.com/tivo.html but when I tried to expand the partition after the copy, it failed with the following error:
Primary volume header corrupt, trying backup.
Secondary volume header corrupt, giving up.
mfs_load_volume_header: Bad checksum.
Unable to open MFS drives.
So I downloaded the MFSLive Linux Boot CD 1.4 from http://www.mfslive.org/
and issued the command mfsadd -x /dev/sda -r 4. Where /dev/sda is the name of the destination copied drive that needs expanding, from cat /proc/partitions.
Everything (edit: did not — see below) work(ed) perfectly. I would probably just suggest following the FAQ/guide for MFSLive Boot CD, using the backup/restore commands.
The drive I bought was the Western Digital 1.5TB green drive from newegg.