Chosen Solution
Just got a new old stock 2017 iMac 21.5 - 3GHz Core i5, 8gb RAM and a terribly slow internal drive. I have cloned the old ssd from my aged Macbook Pro onto a Samsung T5 portable SSD and can successfully boot the iMac upon startup by holding the option key and selecting the SSD, but when I go to the Startup Disk trying to have the iMac automatically use the SSD to boot the computer, the Samsung T5 doesn’t appear in Startup Disk. The T5 shows up in disk utility and it sure looks like it’s mounted and I can access data from it, but I’ve not figured out how to make it the startup disk. I called Apple support (har, har) and they were unhelpful. Anyone here have any ideas? Thanks Jeff
It sounds like you’re hitting a compatibility issue between the macOS your laptop is using (cloned to SSD) and what your new (used) iMac requires and/or the act of cloning messed up the needed boot file on the SSD. How about reformatting your T5 SSD and do a proper OS install on it. Remember you need GUID partition mapping and ideally you want HFS+ file system Vs the newer APFS. That way its more useful on other systems. I would stick with 10.12. 4 Sierra so you can get HFS+ or jump to 10.14 Mojave if you want to use APFS. I don’t recommend using High Sierra. I’m not sure I follow why you need to use the external SSD Vs just reformatting the internal drive and installing a fresh OS on it. HDD’s over time get fragmented so the constant churning of adding and deleting files cause this because of its track and sector architecture unlike SSD which are block based. If you have a good internet connection why don’t you just run the internet recovery service? Reference: Mac startup key combinations
