Originally Posted by zenyou
You can get a SATA/Power extension cable which should work (connect the male end inside the PS3 in place of a normal drive, connect your 3.5" drive to the female end of the extension) for $10-20. Then when you disconnect the drive it is just a bare drive, however, which increases the chance it will be damaged moving it and requires you connect it internally, so you'll have to open up your PC.
Another option would be to get a 3.5" external enclosure for ~$20-40 with ESATA support and a female SATA to ESATA cable (around $5-10) to connect from the PS3's internal header (and the back of the drive bay) to the external enclosure, then plug the enclosure in to the wall. Then you can just move the enclosure between your PS3 and PC, and leave the ESATA converter cable plugged in to the PS3.
Either of these options will require that the drive in question (which won't be in
the PS3, as it can't fit, obviously) is connected to the PS3 for it to function, so they're not exactly ideal.
You're probably better off just buying a larger internal 2.5" drive for your system and using a separate USB drive to move things.