All replies so far were WRONG here - as usually - users with NO or close to zero knowledge giving out the BAD advices!!
Indeed you CAN do that with just only ONE new drive although it would take a bit longer / include additional steps. Here we go:
1. Put the new drive in the external enclosure and format it via your PC in FAT32 (use swissknife or some other tool as Windows are dumb enough not to be able to format the entire drive in FAT32)
Swissknife 3.22: http://www.ps3news.com/forums/attach...chmentid=21939
2. Use the PS3's build in backup function to backup your entire PS3's hard drive to the new drive in the external enclosure. (You might delet some old / unwanted stuff - like old demos and / or some game isntall data - NOT Game save data! - to speed up the backup / restore if you want to).
3. Just to be on the safe side - copy also all unprotected gamesaves to the same drive. You can also copy any other file you want to be sure no to loos - like some picture, music or videos although they've been backed up already in previous step.
4. Copy the entire content of the new drive in the enclosure to your PC - indeed you'd need to have a sufficient space in there but it's jsut temporrary (unless you'd like to hold on the backup files for a bit longer)
5. Now SWAP the hard drive in the PS3. Please note that you'd need to have the FW your PS3 is on downloaded and ready on the USB stick so it could be used after swapping the drives.
6. PS3 will now format the new hard drive and re-install the FW from USB stick. In the meantime - put the old hard drive in the enclosure, format it again in FAT32 and copy over the entire backup content from your PC.
7. Restore the PS3 from the old hard drive uding the build in Restore fuction.
Voila - there you go - new drive in with the content from the old one WITHOUT need for an additional drive