You need the free LBA48 CD
from PTVUpgrade so you can run mfsadd with LBA48 support.
If your TiVo software version is > 5.0 (IIRC) then you should already have a LBA48 kernel on your TiVo, otherwise you will need to run tpip (or copykern) from the same CD to copy the kernel.
However, adding a 250GB drive to an existing 80GB one will most likely have pushed you over the capacity limit for your existing swapfile, so if you get a GSOD in future you may have some remedial fiddling about to do before it can complete.
6023, Lifetime, 240gb, Sky, CC, Mode 0, 100 SPs/AWLs
6022, Lifetime, 250gb, Freeview, CC, Mode 0, 90 SPs/AWLs
601E, my 'test' machine!