That looks like you are hitting the 128 GB LBA48 point.

> dc
10k
268435455
1024/p
262143.9990234375
1024/p
255.9999990463

256
1024*
1024*p
268435456

note that it's the last sector.   Someplace (wd.c?) there's a quirk
table to force LBA48 for that sector.  -current now defaults to this,
I think.   Add a quirk for your drive and try a new kernel.

Before testing with raid, see if you can simply dd the whole disk to
/dev/null.

-- 
    Greg Troxel <gdt@ir.bbn.com>