Hmm - raw external files are incompatible with backing files. Should
we also document that extended L2 entries are incompatible with raw
external files? (The text here reminded me about it, but it would be
the text earlier at the incompatible feature bits that we edit if we
want that additional restriction; compare to the restriction in the
autoclear bit 1). After all, when raw external file is enabled, the
entire image is allocated, at which point subclusters don't make much
sense.
It still may cache information about zeroed subclusters: gives more
detailed block-status. But we should mention somehow external
files. Hm. not only for raw external files, but it is documented that
cluster can't be unallocated when an external data file is used.