There are two possible ways to get around this: -
1. Boot into your system using a rescue disk, edit your silo.conf so that the
bootloader is installed onto sdb, then run silo to install the bootloader.
2. Change the device that the machine tries to boot. From the OK prompt, you
can type: -
setenv boot-device / iommu/sbus/espdma@f,400000/esp@f,800000/sd@1,0
The easiest solution is the second one, but the first is the "proper" way to
do it. Hopefully, if I have guessed the problem correctly, doing either of
the above will solve the problem.