I'd guess you'd be charged the full amount for the replacement HD Unit (and possibly a service call) plus Sky wouldn't be too happy with you. Sky would find out that the unit had been opened when the manufacturer pops it open to diagnose the fault, the serial number on your faulty box could be cross referenced back to your account, so they'd know who removed the disk from the HD Box. I'd say the likelihood is that you'd be caught out. Disks are disturbingly cheap, get a qualifying amount of posts in on this forum and you get free shipping from Scan
You can use copy plus to copy the conents of you old drive to your PC. You don't have to copy the lot, just choose the programmes that you want to keep. Then at the weekend copy from your PC to the new 1TB drive.
My box went doo-lally on the weekend.
It's not the first time either.
It just shuts down and nothing works on it, no remote control acknowledgement LED light or anything.
What I have found is to get it back working I just press and hold the Standby button on the top/front for about 15 seconds, the HDD then spins up and about 10-20 seconds later it responds to the remote and all is fine again.
Must be a similar issue to Windoze BSOD which necessitates a reboot of the PC