The disk-drive monitors itself, to report its "health".
DST is "disk-drive status" checking
Long is the opposite of "short" (one minute is a short time; one day is a long time).
If the short DST reports an error, there is no need to run a long DST. In fact, running the long DST on a "failing" disk-drive may consume the "remaining life" of the disk-drive, pushing it to its "death".
Instead, for a "failing" disk-drive, copy all your personal files, while you still can.
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