> Is our order determined by the OF device list order, or by the kernel
> config file, or what?

Device configuration is done as a depth-first search from the mainbus,
where the order of nodes within a bus is bus-dependent. On macppc, the
mainbus attaches the main PCI bus first, and then any other PCI
busses. PCI attaches its children in order of increasing PCI device
ID. So either your external SCSI controller has a lower device ID than
the internal one, or one or both of them are on other busses (eg obio)
attached to the PCI bus, and the external is on a bus whose bridge has
a lower device ID, or whatever.

There's not much you can do to change the order. But you can build
your own kernel with the drive numbers hardwired. eg:

sd0 at scsibus1 target 4 lun 0
sd1 at scsibus0 target 2 lun 0

or whatever.

-- Dan