First the problem seen when adding on an external or even another internal drive especially an ide model when you already have only sata drives in is that the list of hard drives not the boot order still set at hard drive first then sees the external drive at the top of that list setting it as the default hard drive.
Once you highlight hard drives in the boot order section and press the enter key on many boards the list is brought up where you move the present boot drive back to the top again. You can also change the default drive there if you dual or multi boot different versions of Windows or other OSs on more then one drive.
The "units of measurement" used by manufacturers to indicate the drive's capacity is based on the physical surface capacity of all platters(round metalic disks inside) in decimal form while Windows and other OSs use the binary form there. Most new users and first time system builders who assemble their own cases ask that exact question since it throws everyone off at first! A 500gb drive sees about 465gb available while looking at the properties while in Windows.
When going to buy a drive one useful tool is a capacity conversion calculator seen at The Tomorrow Times: The Hard Drive Capacity Calculator
You simply enter the decimal capacity for the drive and look at the results directly below in binary.