Exercise 2: What the Protective MBR Actually Protects Against — Possible Solution
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Explanation:

Per the chapter's own description, the protective MBR exists
specifically because older tools -- disk utilities, operating systems,
or firmware that only understand the traditional MBR format and have
no awareness of GPT at all -- would, without it, look at sector 0 of a
GPT disk and see what appears to be an empty, unpartitioned disk (since
a GPT disk's REAL partition information lives in GPT's own dedicated
structure, not in a traditional MBR-style partition table). An
MBR-only tool encountering what looks like an "empty" disk could
easily decide it's safe to write a fresh partition table there,
overwriting the actual GPT structure and destroying the disk's real
partition data in the process, entirely by accident.

The protective MBR prevents exactly this: it's a deliberate, minimal
MBR-style entry placed at sector 0 that describes the whole disk as
already "occupied" by a single partition (of a special type ID an
MBR-aware tool recognizes as meaning "don't touch this"), so an
MBR-only tool sees a disk that already appears to be in use rather
than empty, and is discouraged from attempting to write over it.

-- Why sector 0 specifically --
--
-- Sector 0 is where an MBR-only tool would ALWAYS look first, by
-- definition -- that's the one fixed location the entire legacy MBR
-- format expects its own partition table to live. Placing the
-- protective MBR anywhere else would defeat the purpose entirely,
-- since an MBR-only tool wouldn't know to look there; it has to sit
-- at exactly the location such a tool is already going to check.

WHY THIS WORKS AS AN ANSWER
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This explains the specific real-world scenario the protective MBR
prevents (an MBR-only tool mistaking a GPT disk for empty and
overwriting it) and explains why sector 0 is the only location that
makes sense, rather than just stating that a protective MBR exists.