Thanks to both of you. I just saw there were two SATA devices and one is my maxed out hard drive:
WDC WD1600AAJS-40H3A0:
Capacity: 149.05 GB
Volumes:
Macintosh HD
Capacity: 148.73 GB
Available: 2.32 GB
I see now that the 2nd SATA space is occupied by the CD/DVD reader:
HL-DT-ST DVDRW GA11N
I don't think I want to replace that with a harddrive at this time.
I am concerned because my hard drive has gotten completely full (0 KB remaining) a few times, interrupting whatever I am working on, and I can hear faint clicking/grinding noise while it is running (sometimes it is louder than others). I have deleted files to clear 1-2 GB at a time, but I keep filling it back up, and I am guessing it is not good to continue operating at 99% capacity. It also recently made a horrifically loud noise when burning a CD or DVD that was very concerning to me... I was hoping to add a second internal drive so I wouldn't have to backup and reinstall my current one, as I have a number of costly software applications (eg Office 2008, Adobe CD4, Prism, etc) that were just copied from an older harddrive and I have no software installation discs, or codes or licenses to reinstall those...
As a quick fix, I do have the 1 TB Lacie backup drive, so I'm wondering if I maybe I could just change my iTunes music folder location to the backup drive or something to clear an additional 40GB of HD space for now?