Again, you cannot use the unit as per the question you asked. You cannot use it as a OTA or cable PVR to record shows. As mentioned above, you should be able to get into the menu (if you time it right) and playback shows that you have already recorded, without hooking up to the satellite feed. Alternatively, hook up the feed if you can, you don't need an active account to view recorded material.
You can easily pull out the HDD and use it in a computer (standard IDE drive) but with the cost of drives being so low these days you would be better off selling the selling as a proper working unit and buying a HDD (larger capacity and cheap) if you need a drive.
The 5100, 5800 and 5900 are identical units (although there have been some revisionsa to the MOBO). The only difference is the size of the HDD. 5100=40GB, 5800=80GB, 5900=120GB. I converted 3 x 5100s to 5900s by upgrading the HDD.
vmpv - he is not talking about putting another drive INTO the 5900, he is talking about taking it OUT and using it in a computer.