The Update Sequence Array (USA) is a series of uint16 (two-byte unsigned integer) entries that contain the last two bytes of each sector consumed by the structure. Any time we read a sector that is part of that structure, we must account for the USN. First we check to see if the last two bytes are the same as the USN from the structure’s header, to see if we read the sector correctly and to ensure the sector that we read is part of the structure we meant to read. Once we are happy with that, we throw it away. Next we need to step through the USA to the entry that corresponds to the sector we are reading, and substitute those two bytes for the last two of the sector we just read.
