Hi, I am new on this list, but have used Macs since 1989. I am using a 20" Core Duo, and my wife uses a 20" G5, both iMacs. Both have 500 GB SATA II internal HDs. I bought Icy Dock enclosures with FW 400 interfaces and installed 500 GB Samsung SATA II HDs in each. It was not easy finding an external enclosure that had both a SATA II converter board inside (Oxford 924 chipset) and FW 400 interface. I bought both the HDs and the enclosures from newegg.com, my favorite supplier. They work flawlessly. In fact, I cloned the 250 GB internal on my wife's iMac G5 onto the 500 GB in the case before swapping them out. I wanted SATA externals just in case the internals failed, as I back up nightly, and that way, if an internal were to fail, I'd have an external ready to swap. The G5 is user accessible. The Core Duo is not (nominally) user accessible, though I'd cross that bridge when I have to face it.
iMac 20" Core Duo and 13" MacBook Core Duo Mac OS X (10.4.9) loads of peripherals via connected powered hubs; Win XP via Parallels
iMac 20" Core Duo and 13" MacBook Core Duo Mac OS X (10.4.9) loads of peripherals via connected powered hubs; Win XP via Parallels