FYI

My U10 also couldn't boot few weeks ago, with disklabel corrupt error 
message.

I found kernel config file has changed at "wd at pciide" line for ALi 
chipsets.

http://cvsweb.netbsd.org/bsdweb.cgi/syssrc/sys/arch/sparc64/conf/
GENERIC32.diff?r1=1.20&r2=1.21

New GENERIC32 has

wd*     at pciide? channel ? drive ? flags 0x0a00 # Disable UDMA 4

so I changed it

wd*     at pciide? channel ? drive ? flags 0x0f00 # Disable UDMA
                                             ^^^^
to disable UDMA mode completely.

After this fix, I have no trouble about disk access.

--
FUKAUMI Naoki