For several years now, daily, I've ran through a number of different HDD's under relatively stable (although hard-working) conditions (Seagate, WD, and now Hitachi). None of them ever crashed until recently (WDD Caviar Black 640MB). But there have regularly been periods of time (before that) in the past when I would hear what sounded like sharp-twang on Shutdown - like a noise from a toy ray-gun or something. It's happened a few other times in the past, although an HDD has never failed shortly thereafter. So far the only connection I've assumed (without good cause) is that I notice it seems to coincide with an Apple Software Update, I believe from 10.5.4 & up. (10.4.11 to 10.5.4 seems to be OK in spite of that unknown user permissions issue). Mac Pro Wiki Below: http://en.wikipedia.org/wiki/Mac_Pro Question: Should any new/upgraded HDD cache (MB) be no greater than what was programmed in my original System/OS10.4.11 (or BUS?) configuration from Apple (which at that time was an 8MB by default, or 16MB upgrade)? A. Yes, most any newer 32MB cache should be 99% compatible (i.e., Hitachi E7K1000) with both your original 2007 System Build (10.4.11). B. No, I have experience programming for OS X, Unix, I/O, Kernel & stuff... And stick with any new HDD's original matching (MB) cache. C. Irrelevant. Thanks All!