Another idea is to purchase a USB external case for a hard drive, select any hard drive you want (check compatibility of the case with the type of drive of course), and use that for backups. It's not quite as fast as transferring to a second internal hard drive, but it avoids putting two drives (hard, floppy, or CD/DVD) right next to each other. That cuts down on heat problems not only between two adjacent drives, but also in the computer's case overall.
The external USB case (now drive) operates just like a thumb drive in the sense that you turn it on, it shows up at the lower right corner of your desktop (Windows) as an added device, and you disconnect it just as you would a thumb drive (right click and select the option to remove the device safely before disconnecting it). Once it's disconnected, simply turn off the switch on the external case. It cuts power usage and reduces the time the drive operates (only as often as you do backups, assuming you don't do them multiple times per day).
This is a nice solution for the above reasons as well as the fact that you now have a drive you can take anywhere, plug into any computer with a USB port, and you can change the size of the drive later on at internal drive prices, not external drive prices. That alone should offset the cost of the external drive case.