How do I customize a script from "Wicked Cool Shell Scripts"?

This is a fairly straightforward script you've asked about, so this will be an easy explanation.

The most important line in the diskspace script is:

Let's just run a quick df -k so you can see the output in question:

$ df -k
Filesystem              1K-blocks      Used    Avail Capacity  Mounted on
/dev/disk0s9            156277884  63170972 92850912    40%    /
devfs                         101       101        0   100%    /dev
fdesc                           1         1        0   100%    /dev
<volfs>                       512       512        0   100%    /.vol
/dev/disk1s3            156159792 138235824 17923968    89%    /Volumes/X2
automount -nsl [206]            0         0        0   100%    /Network
automount -fstab [214]          0         0        0   100%    /automount/Servers
automount -static [214]         0         0        0   100%    /automount/static

The important thing to see here is that I have one non-root drive, /Volumes/X2, and that it shows up in the list exactly in that format.

This means that you can weed out other drives by using a call to grep. I'd tally up all drives on /Volume by doing this:

You could be even more specific, by using your hard disk name too:

Either way, changing that one line should accomplish what you seek.

In terms of automatically emailing the results every 24 hours, that's a job for crontab. Start with man crontab and you'll find that it's not too terribly hard to figure out.

If you would like to learn more about crontab and related administrative features of Unix, I suggest my brand new fourth edition of Teach Yourself Unix in 24 Hours.