Hi there, I had the same problem and i think i have found a simple workaround for this problem. Probably a little late, sorry about that … I have tested it a number of times on my imac 9.1 and keep getting the same results. I booted my imac (press "opt" key), with a bootable Linux (mint 19) Live USB and found out that the link speed of my Samsung SSD was the full 3 gigabit. Then I made the HFS + 1mb smaller with the linux disk program "Gparted" and then closed linux. Then restart the Mac with macOS and voila the 3 gigabit is now available. (See screenshots) The system will then function smoothly. However, during disk check, macOS indicates a minor error on the disk. I suspect linux writes a value in the partition table which macOS interprets as an error. I have not investigated this further yet. When you fix the "error" in macOS in recovery mode, the link speed becomes 1.5 gigabites again. I admit, the workaround is somewhat unorthodox but I didn't want to keep it from you. Everything works fine so far....