gmirror forget question - why forget gm0 instead of ad4?

[M. Royce]

I want to be sure I understand how to replace a drive if it fails now 
that I have gmirror working. I don't understand the forget command.

I saw this in comments section of Ralf's web page about gmirror:

--------
If a drive fails (eg. drive /dev/ad4):

# gmirror forget gm0
# shutdown -h

replace the drive with a fresh one

# gmirror insert gm0 /dev/ad4

And it should start synchronizing if autosynchronization is turned on.
--------

Why do you forget gm0? I would think if ad4 failed I'd "forget /dev/ad4".

would really appreciate a brief explanation.

thanks!
Royce