fsck.

My external video drive hadn't been the same since we moved houses. (A lot has happened since my last post.) I think I probably improperly ejected a disk while disassembling my workstation. OSX couldn't recognize it for the longest time, and I thought perhaps I broke the cable when disconnecting it from my machine.

Thankfully, there weren't any hardware issues.

diskutil list

I used this command to see if OS X even recognized that something was plugged in. And I did find it, tucked under /dev/diskNum, which helped reassure me that it wasn't actually a hardware issue.

pkill -f fsck

After poking around some more, to no avail (including opening up Disk Utility and trying to Mount, and then attempting First Aid on it), I hit on an error message – "com.apple.DiskManagement.disenter error 0 on external drive" – which had Google pointing me to this forum.

From the forum:

Anyways, if your disk happens to be ExFAT, and you remember improperly removing the disk the last time you used it, chances are, fsck is holding it hostage (metaphorically, of course). fsck is trying to repair the disk, but it will stall forever, so you want to launch Disk Utility, and the Terminal. Try to mount the disk, and switch to Terminal and write sudo pkill -f fsck. The disk will mount in read-only mode, and from there, you can repair it with First Aid in Disk Utility. This won't work for non-ExFat drives, as macOS manages them differently.

• Note: fsck is used to check and optionally repair one or more Linux file systems.

The disk does indeed mount in read-only mode, which seems to mean I can't actually access my videos and pictures until it's been looked over by First Aid, but it's a relief that the data is all still there.

Note here: read up on what the process of disk mounting actually is, and what fsck  was actually doing.