Your PC would only show it as a drive if:
It's a standard drive physically (it's very unlikely it isn't)
It's formatted in a PC-standard way (possibly not)
Go to Start, right-click Computer, select manage. Go to Disk Management, you should see the drive there. It'll probably say it's an unknown format or something.
Don't delete anything, as it will probably contain the firmware for the freeview box as well as the recorded TV.
Windows may be able to do some basic tests from Device Manager, but if it doesn't recognise the format your options will be limited. See what brand the drive is and download a diagnostic tool from them. This will run from a boot CD or USB fob, outside of Windows and will give it a full in-depth test without altering any data on it.
