Hot spares
The risky moment is not when a disk dies — it is the hours the array spends degraded before anyone swaps it. A hot spare closes that gap automatically.
Why the degraded window matters
A RAID-5 array with a dead disk has no redundancy left until it finishes rebuilding. A second failure in that window loses the array. A hot spare jumps in the instant the first drive fails, so the rebuild starts immediately rather than waiting for a human.
Dedicated or global
A dedicated spare belongs to one array; a global spare covers several and is the better use of a drive when you run multiple arrays in one box. Either way the spare has to be the right size and speed to stand in — see RAID and disks.
It is not a substitute for watching
A hot spare that silently activates still leaves you one failure from trouble until the dead drive is replaced and a new spare is in. So the alert still matters: disk health monitoring tells you to arrange the swap, and replacing a failed disk covers the remote hands request.
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