gmirror and disk failure

[Ferruccio Zamuner]

kernel: ad6: TIMEOUT - FLUSHCACHE48 retrying (1 retry left)
kernel: ad6: TIMEOUT - WRITE_DMA retrying (1 retry left) 246989210
kernel: ad6: TIMEOUT - WRITE_DMA retrying (1 retry left) 247031443
kernel: ad6: TIMEOUT - READ_DMA retrying (1 retry left) 50334911

Why such errors make freebsd 8.0 to reboot?
I suppose that gmirror has to mark ad6 broken and bring the mirror in 
degraded mode. Isn't it?

After reboot:

ad6: FAILURE - READ_DMA  us=51<READY,DSC,ERROR> error=40<UNCORRECTABLE> 
LBA=25293375
GEOM_MIRROR: request failed (error=5) . ad6s1[READ(offset=12950175744, 
lenght 131072)]
GEOM_MIRROR: Synchronization request failed (error=5). 
mirror/gm0[READ(offset=12950175744, lenght 131072)]

/sbin/gmirror status
       Name  Status    Components
mirror/gm0  DEGRADED  ad4s1 (26%)
                       ad6s1

If the error is on ad6s1, why ad4s1 has to sycronize with the fault one?