We are seeing more and more systems that have active USB ports that provide power regardless of whether the system is on or not (the feature is meant to charge your cell phone etc. even when your system is off, so that helps). But no matter what, you can "save" which image is active and which one should be used on another PC and then connect to your client machine.
We have 2 zalmans and an IODD and rarely have boot issues. The zalmans are definitely rip-offs of the IODD design, but they work well. Really well.
And then I have a 120 GB SSD in a small enclosure that is partitioned with all the flavors of Mac OS for our mac systems....