Error mounting /dev/ad1s3

[David Niergarth]

Hello,

I'm trying to install the FreeBSD-RC1 ISO image on a 400-MHz G4 (PCI 
Graphics). I use FreeBSD i386 at work so I'm familiar with the 
installation process.

I must be doing something dumb. Everything boots from CD fine, then the 
installer starts up. I choose the standard install option. I configure 
the partitions on my second drive, /dev/ad1 (it starts out as an HFS+ 
formatted disk). I select ad1s3, then choose "A" (auto) to lay out the 
partitions, then exit back to the installer. I continue with the 
installer until it tries to create the root partition. Here's the error 
I'm getting:

   Error mounting /dev/ad1s3 on /mnt: Operation not permitted

Am I missing a step where the slices are set up? I get the same problem 
when trying to install to an external Firewire drive (/dev/da0).

Any advice? Thanks,

--David