The PS3 uses a proprietary format. Your PC can't read it. If you are trying to copy your game saves, use a USB flash drive formatted to fat32. plug it into your PS3 and copy the game saves to it. Now you can transfer the USB drive to your PC and copy the saves to your computer.
If you are trying to backup the ps3 drive, I find it best to use a USB sata dock. Create a small fat32 partition on the drive and plug the dock into the ps3. Use the ps3's backup utility to backup the whole system.