SUMMARY: Booting external disks

All-

  Thanks to all who responded! Most pretty much said the same thing:
make sure the path you are using to boot is correct!

  The problem was that I couldn't figure out how to boot off an external
disk connected to a second controller. 

  The answer is that it is possible to boot off any bootable device as
long as you have the correct path. I had originally tried booting for
the O/S path (/pci@.../sd@3,0:a) but it was pointed out to me that I
need to use 'disk' instead of 'sd'. 

  Also, Mark Neill had a cool trick for getting the pathname correct. At
the ok prompt, type 'show-disks' and a menu will appear. Select the disk
you want -- this puts the pathname into a cut-n-paste buffer. Then type
'boot ^Y' -- when you hit ^Y, the path is pasted in. Wow, neat stuff!

  So, problem solved -- thanks again to all those who responded!

-Eric