You can copy your install from one hard drive to another, they both need to be in the same box though, you can install a 3.5" hard drive into a 5.25" drive bay with an inexpensive hard drive adapter kit, and I've even gone so far as making a set of brackets out of an old case to add an extra hard drive to a box that had no spare drive bays of any size. Once you get both drives in your computer have a look
here for information on how to copy your linux install to the new drive.