I was copying a huge file from a USB external drive when it suddenly disappeared from the Finder. I disconnected and connected it back, but the primary partition wouldn't mount. Rebooting the Mac didn't help.

If in the terminal I type

diskutil list

the primary partition disk4s2 does show up.

It does not show up, however, when I type in

df

because the partition is indeed not mounted.

I'm now trying

sudo fsck_hfs -fy /dev/disk4s2

in and the result is

** /dev/rdisk4s2 (NO WRITE)
Can't open /dev/rdisk4s2: Resource busy

I've already tried

diskutil unmountDisk force /dev/disk4s2

and fsck still won't run.

Full disk access is granted to the terminal in security system preferences.

I'm running macOS Ventura on an M1 Max and the drive is an USB external 8TB "WD My Book 25EE Media" formatted to Mac OS Extended (Journaled) i.e. HSF+.

Any ideas on how to make it mount?