Ever tried connecting a 3.5" SATA HDD to your PS3?
I can tell you it won't work.
The PS3's 15 pin power connection doesn't deliver any +12V since 2.5" HDD doesn't need the +12V, only +5V.
Open up your console and on the backside of the motherboard do the following connection and your PS3's 15 pin power connection will have +12V.
This is tested by me.
Pictures is showing a PS3 Slim, the connection is probably a bit different on the PS3 Fat.
Now you can connect any 3.5" HDD using a SATA 7+15P Data+Power Extension Cable