I've tried to umount a device on HP-UX and got this message:

umount: cannot unmount /dev/device : Device busy
umount: return error 1.

Using command

fuser -cu mountpoint

I've found no process working with the filesystem.

Then I've found the filesystem in /etc/dfs/sharetab as a NFS share.

So then it was easy:

unshare mountpoint
umount mountpoint

and back again

mount mountpoint
share mountpoint

I hope this help.