9100D has NO internal HDD, so if you need one for recording it's absolutely necessary to have a stable one on standby...
A good 3.5" HDD and a good docking cradle is definitely no escape
OTOH a 2.5" HDD is only good for making backups from the STB's internal HDD, which is not the case but instead of the 9500/9600D (ie. with internal HDD)