You need to copy your game save to a usb drive formatted to fat32. Does your game save look like this PS3/SAVEDATA/BLES01863. (folder within a folder)
If your game save came from another person you need to create an account with the same user name as the person you got the game save from or use Rebug's cfw fake save option.
1. Copy PS3 folder to your fat32 usb drive. (if it is a usb pen drive it should already be formatted to fat32)
2. Then take it to your ps3.
3. Go to the "GAMES" column
4. Then go to "Saved Data Utility" (PS3)
5. Click the "USB DEVICE"
6. You should now see your GTA 5 save.
7. Press the Triangle button
8. Press the X button on "Copy"
9. Yes overwrite if you already have a game save
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