Thunderbolt connection iMac

Yes, you can use a Thunderbolt 3 external SSD. If you want extremely high performance you are a candidate for a OWC Envoy Pro EX Thunderbolt 3 and if moderate (still much faster than your current internal HD is) then you would like the OWC Mercury On The Go Pro and OWC On the Pro Go Power Adapter.

davenev wrote:

iMac (Retina 4K, 21.5-inch, 2017)

Yes, you can use an external SSD with your Mac.

In order to provide good SSD recommendations, I must ask what are you planning to do with this external SSD?

Which model year iMac????

Your welcome and good luck.