When a drive spins down it causes slightly more wear and tear on the bearings during the spin up phase than it would if it was just spinning. What the ideal spin down/spin up ratio to always spinning may vary by manufacturerer and I don't know of any analysis that says if the drive spins down for X hours before spinning back up then you'll cause less wear and tear than just always spinning.
Drives that spin down require more time to access than drives that are spinning so you may see the spinning beachball at times (such as open/save dialog boxes or when launching programs) if the drive has spun down.
Drives that spin down use less power.
For people whose objective is to minimize power usage it is better to spin down the drive. For people who want fast data access it's better to leave them spinning.
Most modern drives have mean time before failures that are greater than 50,000 hours (i.e. more than 5 years) and some go up to 300,000 hours (about 30 years). However, that's a statistical mean so nothing says it can't fail with <1000 hours of use (I've had both Seagate and Western Digital drives fail in <1 year in the recent past).
There is no right or definate answer to your question. The bottom line is if you value your data you need a back-up plan that takes into account how critical your data is to you and how critical it is to maintain a minimum down time should a disk fail.
Martin (my name as well): Thanks for your feedback. I just always assumed that spinning the disk down was a good idea, data access aside. But I suppose there are other issues such as parking the head and things like that. For now I think I'll try setting the computer so that they'll spin down if not being used and if the Sonos system doesn't end up with trouble trying to access the music files, I'll be happy. I don't mind letting my main drive spin all the time, nor do I mind letting the Sonos drive spin, but I'd prefer not to let the mirrored backup drives spin continuously.
My backup plan is pretty robust, I think. The two internal mirrored drives back up daily and are CCC clones of my main drive. Should the main drive die, I simply put one of the mirrored drives into the main bay and I'm back up and running. I also have a mirrored G-Tech external drive which operates the same way except that the backups are not sceduled so I do them on a less frequent basis, but both drives get pulled from their enclosure (easy to do from the front with G-Tech's design) and are stored in a detached garage in a fire safe. They're also bootable clones.
Finally, are you aware of any way or any third party software that will allow me to choose which drives spin down and which not to?
I'd prefer not to let the mirrored backup drives spin continuously.
Mirroring drives is not in and of itself, a form of backup. It merely extends the time-to-repair to help keep a drive failure from becoming a data disaster. You are still subject to human error and "crazy software".
You should consider changing that Mirored pair into a rotating A, B backup pair or an A,B,C set (backing up to them alternately).
I understand the limitions of mirrored drives, but my main drive is not mirrored. What is mirrored are my backup drives. One daily backup onto mirrored drives in bay's 3 and 4 in the Mac Pro. And a less frequent backup on mirrored drives that are stored in a fireproof safe in the garage. Should my main, non-mirrored drive fail (hopefully it'll be a long time as it's an enterprise class drive), I've then got any one of four cloned drives to choose from. Of course, I would choose the most recent ones that are internal, but should something catastrophic like a fire occur, I'd then have two identical cloned and bootable drives in the garage.
I recognize that in the case of a real catastrophy, the drives in the garage won't be as current as the drives in the Mac Pro, but in both cases, the backups themselves are pretty robust because they are mirrored. If one backup drive fails, the other will likely be fine and I can then easily proceed from there.
It set up properly, mirrored drives offer some great redundency in case of hard drive failure. What prompted me to go this route was three drive failures in one year, all of which were Seagate Barracuda drives and relatively new. And two of them were backup drives. My backup drives are now Hitachi and mirrored in case one dies. And CCC makes it so I can just plug the backup drive into my #1 bay and I'm back up in business. Except for MS Office which somehow limits the number of times it can be cloned and you have to call MS and get a new product code. Easy to do, but a pain. I learned this as a result of swapping cloned drives in and out of my wife's MacBook. It all worked perfectly until I did it once too many times and MS wouldn't load. Hence the call to MS.
Thanks for all your feedback!
Oh, and any info on controling which drives spin down and which don't?