Is this an old drive? It may be showing signs of wear and tear. I mean, it's weird enough that this happens in the first place, but now it's starting to misbehave in other distributions where it used to work fine so it's even more strange.
But the symbolic links explanation also makes sense, considering how the issue goes away depending on the file manager you're using.
Does it always happen with the same drives?
No hate in this arena, I'm too busy figuring out what I don't know and appreciating you that actually know how to do things WITHOUT the accompanying "oh carp, what did I just do?"
Symlinks (been studying those little demons) sounds quite plausible.
Nothing common among them. I agree with the Symlinks explanation the more I look at it.
Honestly, it may be worth asking over at the Linux Mint forums as well (since you mentioned that this issue also happens with Mint). Hopefully someone there has seen this before and can give some clues?
I've seen this before. Based on what you say here, maybe this is the cause?
If you unplug external drives without unmounting first, the mount point folder is not removed. Since udisks* does not reuse existing mountpoints when mounting, it makes a new one with the number after the name.
exceptions: shutdown and reboot will automatically unmount a drive.
*udisks is the helper utility that mounts and unmounts removable USB drives.
That's bad news 
looks like the forum as a serious user break going on since last week...