Thanks for the lesson!
I managed to easily edit the file and added the ",noatime" behind "defaults".
Wasn't quite sure what was meant by 'real' filesystems, but I tried various combinations, including everything except swap.
Also didn't know whether to expect the changes to take immediate effect or whether a reboot was needed.
No matter- tested everything and did not get a quiet 'disk access' LED.
Did I miss something? Might there be something else I could check on?
- really appreciate the help, guys!
Say, if I get a hang of this Linux stuff, these home-grown NAS systems could be fun to build for friends!

Edit:
Hmm, found this disk spin-down primer that I'll have to read this week.
I guess it's normal for the disks to be spinning all the time with Linux. Weird.
Looks like noflushd is a strong option for me.
Anyone want to argue in favor of disks spinning all day and night?