The disk you tried - it was un-partitioned? Did the Humax format it?
I recall that it does want confirmation to do this - I think that it might be possible via the front panel.
The old drive could be drawing more power than usual - an AV type drive is normally used.
Think the old drive could be corrupt - may be worth deleting all the partitions and reinserting it.
As soon as you get any control over the unit I would reset it to factory default.