Thanks guys. I went a did the crontab thing last night after reading this and went to bed wondering why on earth I'd need to do more than:

0,5,10,15,20,25,30,35,40,45,50,55 * * * * ~/Documents/Scripts/keep_drives_alive.sh

I just checked. My computer was off for the past 15 hours or so. And the timestamps on my .hiddenfile files are up-to-date. So crond did start on boot and ps confirms this:

my-imac:~ me$ sudo ps -ef | grep cron
0 22 1 0 0:00.01 ?? 0:00.01 /usr/sbin/cron
501 528 493 0 0:00.00 ttys000 0:00.00 grep cron

And:

my-imac:~ me$ date
Thu 19 Mar 2009 23:11:09 EDT
my-imac:~ me$ ls -la /Volumes/Storage/.hiddenfile /Volumes/Time\ Machine\ Backups/.hiddenfile
-rw-r--r-- 1 me staff 0 19 Mar 23:10 /Volumes/Storage/.hiddenfile
-rw-r--r-- 1 me staff 0 19 Mar 23:10 /Volumes/Time Machine Backups/.hiddenfile

Computer has been on for about an hour now. Maybe a change of heart on the disabling of crond on boot on Apple's part?

FYI, you can specify "every 5 minutes" in cron with "*/5" in the minute field.

Very nice tip. 15 years using cron and I've been writing it out long hand the entire time.