Can we unmount device?

Hi,

I know that if we need to unmount a device, we use the command umount mount-point, example 'umount /tmp/mount1'
But We can also unmount the device with device name example 'umount /dev/hda6'.

NOTE: I think in RHEL3 we cannot unmount with device name. Correct me if I am wrong.

What is the exact syntax of this?

See man umount. It immediately shows you that it takes devices or files, and that the syntax for unmounting devices is the same as that for unmounting directories.