How to add a slice and partition to a mirror

[Tom Munro Glass]

I have a FreeBSD 7.2 system that had two 37GB mirrored SCSI drives. These 
contained a single slice filling the disk with separate partitions for swap, 
/, /var, /usr and /home.

Yesterday one of the drives had a catastrophic failure - no problem, the 
system still worked on one drive as it is supposed to. I have purchased two 
new 74GB drives because I need more space.

With just the working 37GB drive connected, I ran:

# gmirror forget gm0

Then I connected one of the new 74GB drives and ran:

# gmirror insert gm0 /dev/da1

and the mirror rebuilt itself correctly, but of course only half of the new 
drive is being used.

I've now removed the 37GB drive and run "gmirror forget gm0" again and the 
system is running on the new half used 74GB drive.

How do I define a new slice and partition to fill the second half of this 
drive?

Regards

Tom Munro Glass