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So it was a bit tricky, but I think I got it to work...
1. Save the source disk partition table:
- format c5t0d0
- save (to format.dat)
- format c5t1d0
- fdisk (delete partition 1, exit)
- fdisk (use the default, exit)
- label ( 0 - SMI, use format.dat)
- zpool attach -f rpool c5t0d0s0 c5t1d0s2
- installgrub /boot/grub/stage1 /boot/grub/stage2 /dev/rdsk/c5t1d0s0
- format c5t0d0 and format c5t1d0
- (they should match)