The drive needs to unlocked using either qunlock or diskutil utilities (latter is better) before making a backup if that is what you want to do. Most TiVo gurus do not recommend using two drives for a number of reasons - the main one being the difficultly in diagnosing problems when they crop up. I suggest you just use the 160GB drive and restore it from a backup made from the original drive. Hopefully you are using the PTV version of Mfstools. After the restore run the following commands:
mountcd
copykern <follow the 6 steps this module will lead you through>
then umount and turn off
mountcd
copykern <follow the 6 steps this module will lead you through>
then umount and turn off