In response to a recent blog question we received: Yes, you can install a 5th hard drive into the new 2009 Mac Pro ‘Nehalem’ machines. Simply use the second optical drive bay. The SATA connection is the same for a hard drive as it is for an optical drive and both run off the same system BUS.

Adapter bracket attached to a 3.5" SATA hard drive.
Now, the only issue you will run into is how to secure the drive. Simply put, Tab A is not supposed to go into Slot B. How do you get a 3.5″ drive installed into a 5.25″ bay? You use an adapter bracket.
http://eshop.macsales.com/item/Apple/CLONESLED/
We do carry just such a bracket that will work – originally designed for the Mac clones and PC cases – it does functionally secure your drive in the bay. However, if you use your Mac Pro in an audio-sensitive manner, be advised that suspended metal-on-metal provides no soundproofing. While this will work for those that don’t mind the low hum of a drive spinning, this is not a solution for extra storage capacity if your Mac Pro is intended to be in the same room as your audio recording.

Hard Drive installed into the 2nd optical bay of a Mac Pro
For those audiophiles among you who must have a zero ambient noise environment, don’t fear … we’re working on it. Currently, we’re waiting on delivery of additional bracket systems and when we’re satisfied with their performance, we’ll be offering one or more solutions to you. Watch the blog for developments on this and if you haven’t already, sign up for OWC Larry’s newsletters where the latest and greatest products are usually first announced.
UPDATE (4/17/9) - Well, the good news is that we have found a very elegant solution for installing 3.5″ drives into the optical bay of the Mac Pro that does reduce the hard drive noise as I mentioned back on the 6th of this month. The bad news is we’re still a few weeks out before we’ll have any stock on them. However, you can check out the X-Swing by (((NoiseBlocker))) and even pre-order.
X-Swing installed pics:
The optical and one hard drive installed.
Two hard drives installed.
Note:
Only the above two configurations will fit in the optical drive bay of the Mac Pro. Two hard drives plus one optical drive combined is too large for the space.
Update: Since this article was posted, we’ve come up with several mounting solutions… See our blog post on Drive Adapters Galore—Which Is The Right One? for more information.