The problem here amounts to the fact that we're using the BIOS to load
the kernel.  Since you're booting off the third hard disk that the BIOS
knows about, the BIOS refers to it as device number 0x82.  The boot
block has no way of knowing how BIOS device numbers map to NetBSD device
numbers.  (In the most general case, this is not really possible).  So
it guesses, based on the BIOS device number, that it's unit number 2.
It knows that it's a SCSI disk from what the NetBSD disk label says, so
it constructs a device number for sd2a.

In the `normal' case of having either IDE/ESDI/ST506 or SCSI disks, the
current algorithm is sufficient; it always guesses correctly.  However,
in the case of mixing both types (counting IDE, ESDI, and ST506 as one
`type', from a software point of view), it does not.

In your particular case, you could type `hd(0,a)/netbsd' at the `NetBSD
BOOT' prompt, or you could modify the boot block to always use unit
number 0.