The hard drive in my Late 2012 iMac is starting to die. Since the hard drives in iMacs are not user accessible (at least not without major surgery), I decided to use an external drive as my boot and system drive. I bought this drive, and the HighPoint Dual-Bay Thunderbolt 10Gbps Storage Dock (RocketStor 5212) to house it. I put the drive in the HighPoint, plugged everything in, then...
Booted to recovery mode (Command-R at boot chime). Reformatted the disk to HFS+ journaled (with GUID partition map). Reinstalled MacOS. Once MacOS was installed, I followed the prompts to migrate data from a Time Machine backup (Users, Documents, etc. I clicked all the options).
Everything is up and running great! During the OS install process, the drive format was changed to APFS (instead of HFS+) since I'm running High Sierra.
Lastly, I went into System Preferences and turned on FileVault (which is still running in the background and will probably take another day or two).
The best part: Now my 2012 iMac is even faster than the day it was brand new!