Taken from Microsoft TechNet -
http://technet2.microsoft.com/windo...ed9b-439f-9c31-b93002075d501033.mspx?mfr=true
hope it helps.
A dynamic disk's status is Online (Errors).
Cause: The dynamic disk has I/O errors on a region of the disk. A warning
icon appears on the dynamic disk with errors.
Solution: If the I/O errors are temporary, reactivate the disk to return
it to Online status. For instructions describing how to bring the disk
back online, see Reactivate a missing or offline dynamic disk .
For more information about disk status descriptions, see Disk status
descriptions .
A dynamic volume's status is Healthy (At Risk).
Cause: Indicates that the dynamic volume is currently accessible, but I/O
errors have been detected on the underlying dynamic disk. If an I/O error
is detected on any part of a dynamic disk, all volumes on the disk display
the Healthy (At Risk) status and a warning icon appears on the volume.
When the volume status is Healthy (At Risk), an underlying disk's status
is usually Online (Errors).
Solution: Return the underlying disk to the Online status. Once the disk
is returned to Online status, the volume should return to the Healthy
status. If the Healthy (At Risk) status persists, the disk might be
failing. Back up the data and replace the disk as soon as possible. For
instructions describing how to bring the disk back online, see Reactivate
a missing or offline dynamic disk .