I have a mid-2011 iMac and it runs great mainly due to 32GB RAM...running Linux and Windows VMs (from external Thunderbolt HDD). The overall sore spot performance-wise is the internal (boot) HDD (3TB). I am thinking of getting an external SSD drive to use as the boot drive. To future proof, I want this external SSD to be Thunderbolt 3 and use a TB3 to TB2/1 cable. Since it will be a boot device, should the external SSD be independently powered or is bus powered OK? Would creating a Fusion drive with the external SSD and internal HDD be a bad idea? I know at least the .0 release of macOS 10.13 does not support APFS on Fusion drives. Feel free to chime in with any other information regarding this setup and/or recommended products.