I decided i want to play my ps3 again but 2.5 inch drives for 1-2tb are more expensive. I want to connect a 3.5" sata hard drive to my ps3. if i were to buy a sata to sata plug and leave the 3.5" hanging outside the ps3 would this be a problem and does the RPM matter?
Well there's a risk of physically damaging the drive if it's outside the casing, but as long as it's just sitting their untouched it will be fine.
The drive may also get too hot if it is powered on for long periods without at least passive cooling of some sort.