Safest rsync command options? What are the safest commands to use with rsync to copy from one hard drive, all its content to a new hard drive, both external through my Mac. 

The important thing is I want to be sure permissions and privileges won't risk denying me access in the future if I try to access files on a different computer with a different user account for ex. 

Is rsync -a the answer ? Because I read it conserves permissions and privileges :/. 

Thanks !
rsync -a preserves permissions as far as your current computer/account is concerned. Accessing files from a different user/different account is controlled by a lot of parameters outside of rsync's influence (mainly by the access protocoll and its options used when mounting the drive).