Mac OS X 10.2.8 (6R73) runs on my iBook G3 800MHz. I use the "Put the hard disk to sleep when possible" setting in the Energy Saver preference pane. Something was making my hard drive spin up every 60 minutes. After some digging, I found that SecurityServer was writing to the file /private -> var -> db -> SystemEntropyCache. The following modification disables this write activity; do this at your own risk. You are preventing writing to a file owned by "system".
[robg adds: I concur - this hint is experimental and potentially dangerous. I haven't tested it, nor do I have any intention of doing so! However, if spurious disk activity really bothers you, read the rest of the hint for some advice on how to remove at least one source of activity...]
To stop writing to this file:
cd /private/var/db
rm SystemEntropyCache
ln -s /dev/null SystemEntropyCache
My computer was tested after making these changes. It takes about 10 minutes for the spinning hard drive to spin down. It did not spin up after 2 hours 10 minutes, at which point I manually spun up the drive by opening a file. This was much better than having the hard drive automatically spinning up every 60 min.
Additional notes:
I noticed a process was writing to SystemEntropyCache by using the Finder search to find items of "all" visibility that were modified "today" on the boot volume. I locked the file and changed all of its permissions to "no access". The next hour when the SystemEntropyCache was due to be written, another process spun up the hard drive and updated the /private/var/log/secure.log file.
The secure.log file read (after changing permissions to allow reading):
Jan 13 10:59:42 xs-Computer /System/Library/CoreServices/SecurityServer:
error writing entropy file /var/db/SystemEntropyCache
The computer would then try to write to the SystemEntropyCache every ten minutes. The log file was continuously updated with a new attempt every ten minutes, even after locking and preventing access to the log file. The hard drive did not seem to spin down at all.
Symbolically linking /private/var/db/SystemEntropyCache to /dev/null allows the computer to think it is writing to a file, but the hard drive is never activated because /dev/null is an empty space. Doing this does not break any security features of the OS that I know of. As normal, I was able to: log in, log out, authenticate to do things that require an admin user, and authenticate to access encrypted disk images.
I thought about symbolically linking to a real file with proper permissions on a ramdisk. However, I have found that one way to make the 10.2.8 Finder crash repeatedly is to use the Find command with visibility = all and date modified = today on the boot volume while more than one volume is mounted. I use the Finder Find on the boot volume a lot, so that was not an option for me.
Note that this is just one of many files on the hard drive that is periodically accessed by the computer. You should remove or disable other calendar apps (iCalhelper: inside iCal) and scheduling processes (cron) in order to make sure that your hard drive does not spin up. However, the preinstalled cron processes and iCal may be useful to you. Also note that the SecurityServer will try to write to the SystemEntropyCache every 60 minutes from when it was last successfully written to, not necessarily on the hour.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040114142533291