Exercise 2: Why the RAID 10 Rebuild Didn't Need the Snapshot's Protection, But a RAID 5 Rebuild Would — Possible Solution
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Why RAID 10's rebuild was low-risk:

Per this chapter's own Step 4, "the rebuild here is fast and
comparatively low-risk specifically because RAID 10 recovery means
re-mirroring from the surviving partner disk -- not RAID 5/6's own
much slower whole-array parity reconstruction." During the RAID 10
rebuild, the array's data was never actually at risk of being lost
from the failure itself, because the mirrored partner disk held a
complete, independent copy of everything the failed disk had -- per
fs1-7, RAID 10 "survives multiple disk failures, as long as no single
mirrored pair loses both of its own disks." Even mid-rebuild, only ONE
additional specific disk failure (the surviving partner in that exact
mirrored pair) could have caused actual data loss -- every other disk
in the array remained fully redundant and unaffected the entire time.
Because the rebuild itself was fast and the exposure was narrow, the
practical risk during the rebuild window was low regardless of whether
a snapshot existed.

Why a RAID 5 rebuild would be different:

Per fs1-7's own coverage, a RAID 5 rebuild works completely
differently: "replacing a failed disk requires reading every remaining
disk to reconstruct the failed one's data... a second disk failure
during that rebuild window loses the entire array." Unlike RAID 10's
narrow, single-partner exposure, a RAID 5 array in mid-rebuild has
already used up its only layer of redundancy (RAID 5 tolerates exactly
one failure) -- ANY additional disk failing anywhere in the array
during that rebuild, not just one specific partner, would destroy the
entire array. Since RAID 5 rebuilds can also take "many hours" on
large modern drives per fs1-7, that exposure window is both wider (any
disk, not one specific partner) and longer (hours, not RAID 10's
faster re-mirror) than RAID 10's own rebuild risk.

In that hypothetical RAID 5 scenario, having taken a recent snapshot
(fs1-6) before starting the rebuild would matter much more, since a
second failure during that long, fully-exposed rebuild window really
could destroy the entire array and everything on it -- the snapshot
would at least preserve a separately-tracked, pre-rebuild point-in-time
state (though still not a true backup, per fs1-3/fs1-6's own warning,
since it lives in the same volume group).

WHY THIS WORKS AS AN ANSWER
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This contrasts the specific, narrow failure exposure of a RAID 10
rebuild (one specific partner disk) against RAID 5's much wider
exposure (any remaining disk, over a much longer window), using both
fs1-7's own concept chapter and this chapter's own Step 4 explanation,
rather than a vague "RAID 5 is riskier."