My model of Mac mini has four USB 2.0 ports, one FireWire 800 port, and Thunderbolt. An SSD connected over Thunderbolt seemed like overkill, and I was already powering one display with the built-in HDMI port and another via DisplayPort using the Thunderbolt connection. So any Thunderbolt drive would need to have two ports to support passthrough, and I didn't find any in my price range. (Amazon sells the Thunderbolt/USB 3.0 Transcend 512GB SSD external drive for about $370, including both kinds of cables.)
USB 2.0 is too slow, and I lacked USB 3.0. That made it an easy choice to go with FireWire 800, which maxes out at a raw rate of 800 megabits per second (Mbps). I wound up with the OWC Mercury Elite Pro mini 480GB model, which has FireWire 800 plus USB 2.0/3.0, giving me a path forward to any new Mac with USB 3.0. It's currently $297, including USB 3.0 and FireWire 800 cables. (There are cheaper options for the same capacity with a single interface.)
When the drive arrived, I had to swap some FireWire cables, as I already have another OWC drive attached for backups. I use CrashPlan for incremental and archival backups, and SuperDuper for nightly "smart" clone updates, but I turned to Carbon Copy Cloner for making my migration due to a special feature it possesses.
Here's how I proceeded:
Initialize the external SSD. To use OS X Recovery and other advanced features, you need to use the GUID Partition Table format. (See this tutorial.)
Using Carbon Copy Cloner, I copied my active startup drive in the Mac mini to the external SSD. I needed to keep working during part of the operation, but it fortunately didn't slow the system down much. The copying rate was roughly 40GB per hour, which is fairly slow, but not unexpected.
Because I was using my computer during the clone, I know I missed some changed data. Both SuperDuper and CCC have an incremental clone operation, where only changed files are copied. After the first clone was completed, I quit all active programs and shut down several services, and did another pass. This one took only about 40 minutes to complete.
I used CCC's Recovery HD feature, which lets me copy the recovery partition on an existing disk and then restore it to a new partition after a bootable system is installed.
I renamed the startup volume to a different name, and renamed the external SSD exactly as the startup disk was originally named. Then in the Startup Disk preference pane, I set the SSD as the boot volume and restarted.
After the restart, I had to reconnect to a few services, like Dropbox, which cleverly notes differences in a computer's setup to be sure that someone isn't hijacking your system. I also had to reconnect the CrashPlan backup, by "adopting" a backup from "another computer," a technique it uses for a new machine or a new startup identity. Everything else worked without any fuss.
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