The external HDD is a Western Digital "WD Elements 1TB), I bought the drive in about November 2017 and think it was manufactured in 2017 but I'm not sure of the exact model.
I haven't connected this to the PC while I'm running Windows 8.1, but I have previously connected other external HDDs from WD when I've been running windows and they have been safely removable without remounting. Unfortunately I haven't tested those other drives under linux yet either. I just know that all the WD drives I've used before "dismount" fine in windows and this one doesn't in linux. I can make further checks about that if it could help.
Edited by rp88, 15 January 2018 - 06:08 PM.