promise tx2 unable to rebuild RAID1

[ghtest]

System: 	6.0-RELEASE-p4
PCI-Controler: Promise Ultra 133 TX2
HDD's:		Samsung Spinpoint ATA133 160 GB


RAID1 is degraded,  i've got the following boot messages:
/var/log/messages:Feb 13 16:03:12 xxxxx kernel: ar0: 152587MB <Promise Fasttrak RAID1> status: DEGRADED
/var/log/messages:Feb 13 16:03:12 xxxxx kernel: ar0: disk0 DOWN no device found for this subdisk
/var/log/messages:Feb 13 16:03:12 xxxxx kernel: ar0: disk1 READY (mirror) using ad4 at ata2-master

after adding the spare:
/var/log/messages:Feb 13 16:03:54 xxxxx kernel: ad6: inserted into ar0 disk0 as spare

and the result of:
atacontrol status ar0
ar0: ATA RAID1 subdisks: ad6 ad4 status: DEGRADED

 then i tried:
atacontrol rebuild ar0

and the result of:
atacontrol status ar0
ar0: ATA RAID1 subdisks: ad6 ad4 status: REBUILDING 0% completed

but nothing happens, this messages remains for day's
dd task (/bin/dd if=/dev/ar0 of=/dev/null bs=1m) runs

thanx for any advice