Remember: if you're recording 16 tracks, you'll need storage for those intermediate tracks that apply compression, equalization and mixes. I've been using a 4:1 ratio (raw tracks are given 25% of available space) to let me know I can comfortably work on a project. And you still have to remember to back up and wipe your HD. Just pretend it's a TASCAM cassette recorder and you only own one cassette. Then the necessary procedures become self-evident.
If you're doing a bunch of projects it's convenient to have a CDR as back-up whenever 650 MB of stuff needs that service. Otherwise you'll be fumbling around deciding what file belongs to what project. I usually just create temporary folders right beneath the root directory and monitor their size on a regular basis. When they nearly reach 650 MB, I write them to data CD and wipe the HD after testing the result of the burn.
Some thoughts on recording time available in different HD recording formats- Per MONO .wav track and per 16 MONO source tracks:
16 bit/ 44.1 KHz ---> 2 * 44,100 = 88,200 Bytes/second/track = 5292 KB/ min
24 bit/ 48.0 KHz ---> 3 * 48,000 = 144,000 Bytes/second/track = 8640 KB/ min
24 bit/ 96.0 KHz ---> 3 * 96,000 = 288,000 Bytes/second/track = 17,280 KB/ min
32 bit/ 96.0 KHz ---> 4 * 96,000 = 384,000 Bytes/second/track = 23,040 KB/ min
5292 * 16 tracks = 84,672 KB/ min : 236 minutes on 20GB / @25% = 59.1 minutes
8640 * 16 tracks = 138,240 KB/ min : 145 minutes on 20GB / @25% = 36.2 minutes
17,280 * 16 tracks = 276,480 KB/ min : 72 minutes on 20GB / @25% = 18.1 minutes
23,040 * 16 tracks = 368,640 KB/ min : 54 minutes on 20GB / @25% = 13.6 minutes