A script that outputs basic usage information for a single hard drive:
DRIVE-NAME • AVAIL-SPACE / TOTAL SPACE • USED-% / USED SPACE
I use four copies of the 2 scripts attached. In the second script, if you replace 'disk1' with 'disk2', 'disk3' and so on, you can monitor other drives as well.
In the first, I hard-coded the name of my main drive, for laziness. I also used powers of 1024 to calculate total drive space for display purposes as I didn't like looking at 995MB as opposed to 1TB. Adjust if desired.
Each of the others monitors disk1 through disk3, displaying the same information. These scripts use powers of 1024 for total drive space calculations as well, but only to GB. Adjust if desired.