My iMac has only one 1TB internal HD. Through USB3.0, I have then decided zuzulegen an SSD. I am thrilled cloned system on the SSD and there you go. I reach about 410 MBytes per second, the internal HD creates just 120. And then the faster access times, so an external SSD is also worth the iMac definitely. As external case I would recommend from the Inateck FEU3NS-1E but it respects the her the SSD optimized version take.