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

I'm now trying

sudo fsck_hfs -fy /dev/disk4s2

in the terminal 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.

Any ideas on how to make it mount?