When I refer to a shell, I'm talking about an enclosure that includes the connectors inside to hook up a hard drive. You can purchase ready to run USB compatible external hard drives, but they cost more than they're worth. If you already have a spare hard drive laying on a shelf, you just buy the USB "shell" and install your own drive. At most, it's just a matter if removing a couple of screws, inserting the drive, hooking up the connectors, then putting it back together. Most have their own power cord, so you just plug in the power, flip a power switch, then connect the USB cable. Your PC should treat it the same way it treats a flash memory drive.
If you have a spare drive on the shelf, you just need to know if it's ATA/IDE or SATA - the connections are different so you need to buy the appropriate enclosure. Newegg has a few dozen to choose from.
If you have a spare drive on the shelf, you just need to know if it's ATA/IDE or SATA - the connections are different so you need to buy the appropriate enclosure. Newegg has a few dozen to choose from.