cgsecurity.org

Testdisk seems not to backup full image. Image is missing sector 0 so later trying to work with this image it doesn't find any partition which did while working with drive directly.
How to save whole disk?

Use ddrescue as described in the manual to backup your disk.

HxD also worked fine to save full microSD image. Not sure why testdisk skips first sector.

The backup of Testdisk is primarily partition-related. Testdisk does not pretend to backup a disk.