They cannot usually be recovered, i'm not sure which model of box they have, but if you do not connect the old box to your co-ax feed (which will de-activate the channels anyway) you can still fire up the old box without a live feed and watch your stored programmes, that was the case with an old Cisco box, at least..
Definitely sounds like the Hard Drive is dying though, i'm having similar issues at the moment!
Great news they're sending you one though! The most you'll have to do is log on to your account, or call 150 and request they send the "magic signal" lol
Kind regards,
Lee