atacontrol weirdness: RAID 1 array is shown to be degraded but both disks are up

[Gabriel Ambuehl]

The 4.9 box has got onboard Promise RAID which so far has worked just
fine.

However, today I got a report about a degraded disk array so I went to
check and found something rather odd:

$ atacontrol list
ATA channel 0:
    Master:      no device present
    Slave:       no device present
ATA channel 1:
    Master:      no device present
    Slave:       no device present
ATA channel 2:
    Master:      no device present
    Slave:       no device present
ATA channel 3:
    Master:  ad6 <WDC WD800BB-00CAA1/17.07W17> ATA/ATAPI rev 5
    Slave:   ad7 <WDC WD800BB-00CAA1/17.07W17> ATA/ATAPI rev 5
$ atacontrol status 0
ar0: ATA RAID1 subdisks: ad6 ad7 status: DEGRADED


How can a RAID array be degraded when both disks are still there? How
do I figure out what disk holds the current set of data? And
what do I need to do, anyway?