Several days ago I was adding a second drive (a speedy SSD) to my Mac Mini. After it was reassembled, the OSX 10.9 Mavericks install drive inserted and restarted, I navigated to disk utility to format the old HDD and the new SSD prior to installing Mavericks. An error message appeared indicating the “Fusion Array is broken” and “Should I repair it?”
A “Fusion Drive” is two physical drives and software, a) a logical volume manager that spans multiple disks into a single volume, and b) the piece that Apple is really proud of, a tier manager that manages data transfer and storage between the speedy SSD and laggard HDD.
After responding “Yes,” Mavericks created its Fusion Drive and installed itself. Fusion Drive introduces a small speed degradation, but it’s minimal for the work I do and the single volume architecture is a nice feature.
While pleased about the Mac Mini, I had never seen this behavior on the older iMac. It has an SSD and HDD but they’re two stand-alone drives. Why didn’t it “fusion” itself like the Mini? A little Google-ing provided the answer.
Initially, one of the iMac’s existing drives refused to unmount preventing the fusion initialization from running. The solution was to boot the iMac using an Ubuntu Live CD and forcibly unmount and repartition both disks. That had the undesirable effect of erasing the Bluetooth pairing information Apple keeps in a hidden disk partition, but an USB keyboard and mouse remedied that. Old School!
Today both Macs are “fusioned.” Fusion isn’t a cache or hybrid disk with a tiny SSD. It’s more like Intel’s SRT except that Intel manages small blocks while Apple manages whole files. I could see a drawback if I ever work with 4G files (Apple’s protection area) but that’s unlikely to happen anytime soon.