Chosen Solution

What will I need to connect this external SSD to the iMac’s Thunderbolt 1 port ? It’s currently connected to the iMac’s USB 2.0 port and it’s S L O W

Sorry to say thats the limits of the USB 2.x port you are using in this system. Your only options is to use a Thunderbolt 1 or 2 drive via the systems Thunderbolt 1 port. Just think how hard it was for NASA trying to recover old moon images! Digital Recovery of Moon Images