The "Disk Doctor", as shipped with AmigaOS versions 1.2-2.04 was both a crude and cruel tool.
Its primary purpose was to "restore" a damaged volume to a state which enabled read-access, allowing you to copy the data to a different volume. At the time most users (myself included) viewed it as a repair tool which should have been able to restore a damaged volume to operational state again. You needed a second floppy disk drive to be able to copy the data to a different disk which was a rare thing to have in the old days. So we settled for what should have been a "fix in place" repair operation, but in reality "Disk Doctor" never left a volume in a better state than it was before, even if the file system structures had been sound to begin with.
The reasons why the "Disk Doctor" was so bad at repairing anything are legion.
For example, if a track could not be read because of a flipped bit or physical damage, "Disk Doctor" would attempt to restore the entire track to a sane state by reformatting it (physical low-level initialization). The "Disk Doctor" uses the same buffer for reading and writing data, which means that when it reformats a track, it will (as a side-effect) write the data that was last read back to disk. That data was written to a different track, and if it happened to contain file system data structures, then "Disk Doctor" would later pick them up and try to make sense of them: it was basically "fuzzing" itself.
Part of the diagnostic operations performed prior to "repair" was to detect damaged or deleted file system structures, such as files or directories which no longer had a valid parent directory ("orphans"). The "Disk Doctor" would gather these and add them to the root directory. It did not check if there already were files or directories present in the root directory which shared the same names, which could have the effect of corrupting the root directory. You might be able to list the root directory and find several files with the same names stored in it, but accessing these would only go as far as the first entry in the directory entry list.
When trying to recover the "orphaned" directory entries the "Disk Doctor" made no attempt to verify that the files and directories it added to the root directory were structurally sound. You could have ended up with fragments of files which were deleted ages ago and whose parts had been partly overwritten since they had been deleted. These broken file data structures could reference data and metadata blocks, which in turn would then break the disk validation process.
