Choice of external SSD - iMac 27' late 2013 - advice needed.

[KOTULCN]

I didn't even install Boot Camp in the internal SSD; I booted from a Windows 8 USB stick over UEFI.

That may have been the reason. Maybe it'll work after I install a tiny 20GB Boot Camp partition in my internal drive first.

Here is the thread that describes how to install correctly.

https://forums.macrumors.com/threads/1653997/