Sara, this is a great advice for shrinking backups after they're created, but this thread is about backup time at image creation.

There's another possibility, by using the "sparse" option of dd:

serverfault.com/questions/439128/dd-on-e...t-want-empty-portion

However, i didn't test it (yet), but seems worth a try if you have some time to enter commands and let the system do the work.
Append calls of "date" pre and post execution like so:

date && someothercommand && date

It will echo start and finish date/time before and after someothercommand is being executed. So you can leave the screen while dd is running.
Writing back the dd image to sdcard would be best with omitting the "sparse" option, i guess.

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