Avago Technologies MegaRAID SATA 150-4 (523) - Manual (Page 33)

RAID Components and Features 2-11
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Hot spares are used only in arrays with data redundancy, for
example, RAID levels 1, 5, 10, and 50.
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A hot spare connected to a specific RAID controller can rebuild a
drive that is connected to the same controller only.
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You must assign the hot spare to one or more drives through the
controller BIOS or use array management software to place it in the
hot spare pool.
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A hot spare must have free space equal to or greater than the drive
it would replace. For example, to replace an 18 Gbyte drive, the hot
spare must be 18 Gbytes or larger.
2.4.13 Disk Rebuilds
When a physical drive in a RAID array fails, you can rebuild the drive by
recreating the data that was stored on the drive before it failed. The RAID
controller uses hot spares to rebuild failed drives automatically and
transparently, at user-defined rebuild rates. If a hot spare is available, the
rebuild can start automatically when a drive fails. If a hot spare is not
available, the failed drive must be replaced with a new drive so the data
on the failed drive can be rebuilt. Rebuilding can be done only in arrays
with data redundancy, which includes RAID 1, 5, 10, and 50.
The failed physical drive is removed from the logical drive and marked
ready awaiting removal after the rebuild to a hot spare begins. If the
system goes down during a rebuild, the RAID controller automatically
restarts the rebuild after the system reboots.
Note: When the rebuild to a hot spare begins, the failed drive is
often removed from the logical drive before management
applications detect the failed drive. When this occurs, the
events logs show the drive rebuilding to the hot spare
without showing the failed drive. The formerly failed drive is
marked as ready after a rebuild begins to a hot spare.
Note: If a rebuild to a hot spare fails for any reason, the hot spare
drive is marked as failed. If the source drive fails, both the
source drive and the hot spare drive are marked as failed.
An automatic drive rebuild does not start if you replace a drive during an
online capacity expansion or RAID level migration. The rebuild must be
started manually after the expansion or migration procedure is complete.