You can copy save files for games specificly from the PS3 HD without doing a full backup/image of the PS3 HD
Wouldn't hurt to make a folder or partition to store the save files in on the portable USB HD.
- Destiny - The Taken King (PS4)
- Elder Scrolls Online Tamriel Unlimited (PS4)
- Assassin's Creed Syndicate
- Fallout 4