Backup only "filled" (=used) sectors?

[pstein]

I have a 32GB USB flash drive.

This device is used by only 15% (as indicated e.g. by partition management tools).

However when I use USB image tool to create a backup then the resulting image is (uncompressed) 32 GB and compressed approx 21 GB.

How comes?

Is there a mode which backups really really only occupied sectors and omit free sectors?

Peter

[Alex]

Device mode backups every byte of the device, regardless the partition data.