User Thomas said in an earlier thread on A.org about this error:
"Error 21 (not enough sectors found) means that you either try to read beyond the end of the disk (is it larger than 1.6gig ?) or the disk has a real physical read error (magnetical fault).
In the first case you should check and adjust the HDD size in HDtoolbox. In the latter case you should buy a new hdd and trash the old one. Before you do that your could try a full format or even a low-level format.
Bye,
Thomas"