If target sectors are missing or otherwise unreadable, the defragmentation silently stops (without an error message!), leaving the disk in corrupt state.

For example, here is disk with missing Cylinders 1 and 2, and a single file that, therefore, cannot be moved.

After starting the defragmentation, the single file on the disk disappears.

This issue should thus:

• fix this problem,
• add error message in case of problems,
• possibly introduce a smarter way of defragmenting a disk (though this is maybe a topic for another issue than this one).