Ofcourse I'm running from /programs0, but here is the relevant proc/nn info. My guess is that at the end of a sync, the software actually does "umount /drive0 ; mount -oro /drive0" instead of "mount -oremount,ro /drive0" and the umount fails because the executable image still has a reference on the filesystem.
(btw. I was logged in with telnet, which explains the two sockets and /dev/ptyp0)

sh-2.03# ls -lR /proc/12 

/proc/12:

lrwx------    1 0        0               0 Jan 25 17:37 cwd -> /

lrwx------    1 0        0               0 Jan 25 17:37 exe -> /programs0/telnetd

lrwx------    1 0        0               0 Jan 25 17:37 root -> /



/proc/12/fd:

lrwx------    1 0        0              64 Jan 25 17:37 0 -> /dev/ttyS1

lrwx------    1 0        0              64 Jan 25 17:37 1 -> /dev/ttyS1

lrwx------    1 0        0              64 Jan 25 17:37 2 -> /dev/ttyS1

lrwx------    1 0        0              64 Jan 25 17:37 3 -> socket:[3]

lrwx------    1 0        0              64 Jan 25 17:37 4 -> socket:[21]

lrwx------    1 0        0              64 Jan 25 17:37 5 -> /dev/ptyp0