is this a sparc ultra 5 or sparcstation 5. big difference.
if it is a spacstation 5 i dont believe it can even run solaris 10. I have a sparcstation 5 and it has trouble with solaris 8. i run netbsd on it and it is find. as for the boot disk, if it is a sparcstation5 the boot cdrom needs to be scsi device 6 on channel 0. if it is an ultra5 then i honestly dont know, what is causing your problem, since i havent worked with any ultra hardware.
good luck