If it's a USB HDD then it'll work but before connecting it to your PS3, you have to convert it to the FAT32 file format otherwise the PS3 won't recognize it !
Here's how do it !
First Connect the external hard drive to your computer using your USB cord.
Open your "Start" menu and click on the following string of menu options: "Control Panel," "Administrative Tools," "Computer Management" and "Disk Manager."
Select the external hard drive and give attention to the type of "File System." If it reads "NTFS," then right-click it and choose "Delete Partition." This will ensure the partition you use is "FAT32." If it's not "FAT32," then proceed to Step 4. If it is "FAT32," then move on to Step 5.
Run an Internet search for "CompuApps Swiss Knife." You'll find the official website within the first few search results. Simply enter the site and download the free application. Once downloaded, install the program and run it. When you're inside the program, choose the external hard drive that needs to be partitioned. Configure the settings in the following manner: "Partition Type-Primary," "File System-FAT32" and "Volume Label." Choose "Create" and your FAT 32 partition will be set up. Proceed to Step 5.
Open "My Computer" and click twice on your empty external PS3 hard drive. Right click within this area, move your cursor down to "New," and select "Folder." Name this folder "Photo." Repeat this process to create three more new folders: "Music," "Video" and "Game." This will ensure the folders on your external hard drive are identical to those on your PS3.
Click the green arrow in the bottom right corner of your screen to remove and choose "Safely Remove." Unplug your external PS3 hard drive once a message appears indicating it's safe to do so.
Connect your external hard drive to your PS3 and enjoy.