All this speculation about DMA and boot sectors and whether Plan 9 is actually installed is really enthralling, but it's not John's problem.
John's problem is that reading or writing a certain part of the disk causes an i/o error that the disk driver can't seem to recover from. It could be a disk problem but it could also be a driver problem. Without making kernel changes to turn on some debugging prints in the sdata driver, it's hard to say which. Probably the SATA card or disk is slightly non-standard and behaving in a way that confuses the driver. You might try running a whole disk diagnostics program from the manufacturer (the kind that come on bootable media) just to double-check that the disk itself doesn't have some bad sectors. But it really sounds like the disk just doesn't behave quite according to expectations. The reason /dev/sdC0 looks okay after the i/o error is that it is updated by a user-level program that runs at boot (disk/prep -p and disk/fdisk -p), so even if the disk goes south underneath, the kernel will remember the configured partitions. Russ
