I'm just curious if anyone else has decided to jumpstart your old HDD iMac with an external SSD? I know there are many threads about doing a HDD for SDD swap. But, after doing a bit of research on the topic, I decided the simplest way for me to give my late-2013 iMac a second life with zippier performance was to simply buy an external SSD and run MacOS from it. Then it was a matter of settling on a specific drive. It seemed that all signs pointed at the Samsung T3 SSD as the option to go with so here I sit with Sierra running from an external SSD sitting on my iMac stand. Specifically, I re-formated the Samsung T3 as Mac OS Extended (Journaled) with GUID Partition Map. And from my USB thumb drive that I had used to previously do a clean install on the HDD, I erased and renamed the SSD and went about installing Sierra. I also, out of OCD issues, completely erased the Macintosh HDD. I may re-install MacOS on the HDD but probably not. Not to worry though, all of my documents, photos, videos and music are backed up in iCloud Drive and on an external SanDisk card. Double coverage. I must say, after a couple of hours of running MacOS this way, I feel that my iMac is finally performing the way I would want it to. In our house we have a 2014 rMBP and a 2015 MBA so I'm familiar with the zippiness of SSDs. But not with the iMac. Now, I feel like I have a machine I can get my money's worth well into the future.