[BenjiWiebe]

Next time you have a disk where you need to do repeated dd runs over different ranges, or suspect that you might need to, use ddrescue. It tracks which sectors have been recovered (and has lots of useful options).

You can also get 'partclone' to generate a list (in ddrescue format) of sectors containing data, so you don't need to try to read unused areas of the disk. For the partclone trick to work, the FS does need to be at least somewhat readable.