Okay
So. I have never seen a 3.5" drive that was bus powered. The 2.5" are great portable drives. I have an 80GB I assembled myself, that is firewire bus powered. Great for trips with the laptop. The guy asked about platters, not disks. Most drives nowadays have 2-4 platters I believe, used to have more, but higher densities have allowed fewer.
The issue with LaCie is you don't know what kind of drive you are getting. One week it could be Western Digital, another Maxtor, then Seagate. That being the case really no way to know how many platters, because each company is doing its own thing.
Also I work in film post production for a university. We have been having LOTS of d2 failures. usually a chipset failure that then corrupts the drive structure. About 50/50 to our success rate on recovery. We are now recommending the LaCie porsche drives or the Seagate externals or the Western Digitals.
Lastly a plug for macsales. They are great, and will take care of you. So just my couple cents. best of luck.
oh lastly, you could get a 3.5" enclosure that has a built in power supply, so you wouldn't have an external power brick, make cords a little cleaner. Just a standard power cable, and a firewire cable.