In theory, you should have no problems. Any 2.5" drive should fit in that bay. It's good you have the system on the 12 GB flash, it'll make booting after installation of the HDD (drive) easier. Now, the Slim drive will fit: the Slim (S) and SuperSlim (SS) both use 2.5" SATA (I think they're all Hitachi SATAs). They even use the same SATA cables (I believe a SATA-III and 15pin power). You can even plug and play the drives, provided that the OS version on the boxes are exactly the same. If the versions are different in any way, you will need to reformat that drive. But, I know a way you might be able to save the data on the drive if you have the right tools.
Now, afaik, the bay for the SuperSlim (SS) exists even on the PAL version, because they're trying to get you to by that 250 GB expansion, I'm sure. I do not know if the cables are in the your SS's bay; I doubt it. But, you should be able to use the cables from your friend's box. I believe there is a type A (male) connector on the board and the drive so the cables are B-B (female-female). Don't quote me on that. The SATA cables should be attached between the motherboard & the drive; just yank them out (carefully) and plug it into the connector ports that should be there in your bay. Close up your box, and the PS3 should boot from the flash drive. If it does not boot, it's trying to boot from the drive you swapped. You'll have to unplug that guy and try something else.
Here's what you'll need to do if it tries to boot from the swapped drive:
- Get a SATA ↔ USB adapter if you don't have a SATA port on your computer.
- Plug in the swapped drive.
- Run a disk utility. If you don't have one, I can step you through a CMD prompt on Windows or a Unix shell. Let me know.
- If you want to backup the data make a copy of the disk onto a local drive.
Files can only by 3.9 GB in size (requirement of the FAT32 system PS3s use), so make sure you specify that in the disk utility options or... I'll figure out a way to run that from the shell if you ask (please don't).
- Reformat the drive as a FAT32 drive. Make sure this is the only partition on the drive.
After the HDD is reformatted (might take a while) create the following directories on it (case sensitive)
- Cut the data from your backup to the appropriate folder above.
- Eject the drive, then install it in your box.
You should now be good to go.