recover data after using dd command

Solution 1:

Quite simply, its probably impossible if dd finished its work -if you stopped it partway, any data that wasn't overwritten should be recoverable. Testdisk is most useful when a system got formatted - and this usually involves 'marking' sectors as not in use, rather than erasing them. With modern drives, despite what the guttmann paper says, overwriting the data is extremely likely to leave data irrecoverable. DD overwrites data and as such, there's no practical way you can recover your data, short of recovering off backups.

Unfortunately, forensics is tricky. photorec is a good start, but you probably need to try things like scalpel and foremost too. If the data has value, its probably worth getting a quote from a professional drive recovery company too.