Booting from Thunderbolt on 2011 iMac

Hi Folks:

I have a friend's 2011 iMac which has become really slow running Sierra. I thought to increase speed for her by moving everything to an external SSD so I used CarbonCopyCloner to do that. The disk is a new USB 3.1 Samsung 500GB SSD drive. I made it bootable and plugged it into a USB port on the iMac. It booted fine, and was indeed much faster, but those are USB 2.0 ports...I guess the 2011 iMac came out before USB 3.0. I knew I could get 10x the speed by using the iMac's Thunderbolt port (Thunderbolt-1, I believe), so I plugged into two different Thunderbolt-to-USB hubs: one unpowered (the StarTech Thunderbolt to eSATA with USB 3.0 adapter http://a.co/7YsUrAm) and a powered hub (my Elgato Thunderbolt 2.0 dock http://a.co/aeTrEJi), but it just won't boot. When I hold down the option key at startup, only the old internal drive and its recovery partition show available, not the new SSD drive.

So, to summarize, it boots just fine plugged directly into a USB 2.0 port, but not when plugged through an adapter or dock to the Thunderbolt port. I can't figure out why, unless Thunderbolt 1 didn't support booting? Ideas?

Thanks!