here is a pointer to PWAIT$SDA from

If SDA> READ SYSDEF also hangs, you have more than one problem ! This
command just reads a file from your system disk.

Note that you would most likely have to reboot anyway, even if you are
able to find out, which MUTEX your process is waiting for. If so,
consider to take a forced crash, this would at least allow you to
continue the hang analysis in your spare time ;-)