Fortunately I use CCC religiously. I have an eGPU with my 2012 Mini (a Mantiz Venus with RX 580 graphics) and I do some gaming on it. Today I dutifully downloaded and installed the 10.13.4 update. Oops. Rebooting did nothing but leave a display that had the power light blinking. I put the HDMI output back into the Mini itself and got the video back. Then I did some googling (should have did that first) and learned that whether intentionally or not (my vote is intentional), the "official" eGPU support in 10.13.4 breaks eGPUs in TB1 and TB2 systems. Fortunately, CCC to the rescue -- I booted again on a spare, cloned volume and restored last night's backup to my primary boot drive. It's working again, back to 10.13.3. But if you run an eGPU on less than TB3 hardware -- do not install 10.13.4! Stay tuned to further announcements, both from Apple and from the "community" which finds ways to make things work. Maybe it will work again (apparently, one seeding of a previous HS beta broke that but Apple fixed it), maybe Apple has put their foot down to try to make these folks buy new Macs with TB3 (as an aside, give me a quad-core Mini with TB3 and take my money), but whatever the case, the new 10.13.4 update breaks eGPUs for Macs with TB1 and TB2.