Here is output from a broken mirror in syslog:
Jul 10 00:56:41 #host kernel: ata3: SATA link down (SStatus 1 SControl 300)
Jul 10 00:56:41 #host kernel: ata3: EH complete
Jul 10 00:56:41 #host kernel: ata3: exception Emask 0x10 SAct 0x0 SErr 0x4040000 action 0xe frozen
Jul 10 00:56:41 #host kernel: ata3: irq_stat 0x00000040, connection status changed
Jul 10 00:56:41 #host kernel: ata3: SError: { CommWake DevExch }
Jul 10 00:56:41 #host kernel: ata3: hard resetting link