The external drive does not show up in First Aid on all machines?
I'm reading folk complain about Thunderbolt to drive issues. You won't be the first or last it seems. Read https://www.google.com/search?q=apple+thunderbolt+connected+drives+don't+show+up for more ideas.
I have a 1TB Seagate Expansion Portable Drive that I have connected successfully to my laptop (MacBook Pro 2016 running High Sierra 10.13.4) in the past but yesterday when I plugged it in it would not show up. The hard drive works fine with an older laptop that I have at work (MacBook Pro Early 2015 running El Capitan 10.11.6) so I know it is not the cord or the drive. The only difference is that I'm connecting it to my home laptop using a Hyper Drive Hub since that laptop has thunderbolt ports and the one at work doesn't. I checked to see if something was wrong with the hub by plugging my camera into it via USB and the laptop was able to read the camera's memory card just fine.
So far I've tried shutting down/unplugging the laptop and restarting it but that didn't work. The drive does not show up in Disk Utility on my home laptop so I ran the First Aid using the older laptop that I have at work. Nothing came up. Not sure what else to do. Any suggestions?