dd disk imaging - which conv switches to use and when?

Question

I will be backing up a large (750GB) disk to an external USB disk using dd.
What is the most appropriate use of notrunc, noerror and sync conversion arguments? It seems some people use them in different ways, or not at all.

Also, what is the best block size? USB is likely to be the bottleneck here.

dd if=/dev/sda bs=1M | gzip -c > /mnt/sdb1/backups/disk.img.gz

gzip -dc /mnt/sdb1/backups/disk.img.gz | dd of=/dev/sda bs=1M

Thanks.

Answer 1

Avoid using dd; programs like ddrescue have a much better status reporting, imply noerror, and conversion is not desired on disk images at all.