Apparently Full External HD Won't Mount, Freezes MBP

I own a mid-2014 15-inch MacBook Pro running on macOS High Sierra. I was exporting a finished FCPX project as a Master File (Pro Res 4:2:2) which was being saved to an external hard drive that had given me no previous issues. I hadn't checked to see if there was enough room on the drive to store the rendered file. At some point the exporting process froze and a pop up appeared stating that the "Share" had failed. The spinning "beachball of death" had also appeared, and time, attempts to "Force Quit" did not work. So I had little choice but to do a hard shut down of the computer (holding down the Power key).

Since then, the computer runs fine. Another external HD that was attached to the computer, and held the actual FCPX project being exported, works fine. But any time I try to mount the external HD to which the rendered master file was being exported the drive won't show up on the Desktop, and I quickly get the spinning "beachball of death" whereby the computer completely freezes up. I have managed to get the drive to appear, greyed out, in Disk Utility. If I act quickly enough to check the drive's info I see that there is 0 GB space available. But the "beachball" quickly appears. I cannot do First Aid, unmount the drive, etc.

Does anyone have any thoughts about how I can get the drive to mount or at least be viewed by a disk utility of some sort without freezing up the computer?