Now I've been to my niece's and I've been able to access my OMP with their Wi-Fi password which didn't include any hashtags, not available on the OMP keyboard. Alas, it looks like the OMP hard drive has died (see attached screenshots). As I don't think it's worth it to install another hard drive, i'll probably be giving it to a nephew to tinker with.
On the subject of the external drive with miscellaneous files (EMF), a strange thing happened when I connected the EMF to a great-nephew's 2017 Macbook Air, which happened to run High Sierra.
The EMF showed on his desktop, but when I tried to double click it open it wouldn't open, so I disconnected the EMF and reconnected it but now it wouldn't show. This I did several times and every time it wouldn't show on the 2017 MacBook Air. Should have simply highlighted the folder when it appeared the first time, and copied it into iCloud or Dropbox! Ah well, I'll put that down to lack of experience with these things.
A nephew says he can extract the data, although he doesn't have a Mac, so maybe I can get the contents out anyway.