question about backing up my legit bluerays

hi all. i have been trying to backup my legit blueray disks but not having any luck due to the small size of disk space on ubuntu. i have read b4 to use the command Dd if=/dev/cdrom of=/movietitleBlueRay.iso. but im not getting any luck with that. im not the best at linux cuz i have been trying to get used to it. can some 1 tell me a command to have the blueray rip to a thumb stick or external hdd. wouldnt it be some thing like this
Dd if=/dev/cdrom of=/dev/disk2/movietitleBlueRay.iso? any help would be great thanks guys