/dev/hdb is the whole drive
/dev/hdb1 hdb4 etc is a single partition.
You should be restoring to a drive (hdb), not a single partition (hdb7)
Your restore line should be something like:
mfsrestore -s 300 -r 4 -zxpi /mnt/dos/tivo_25_turbonet.bak /dev/hdb
Also if that's a really old image made with mfstools 1.1 then you may need the -b switch in there too:
mfsrestore -s 300 -r 4 -zxbpi /mnt/dos/tivo_25_turbonet.bak /dev/hdb