Q: 3.5" internal
A: The S stands for Seriel. The P for Parallel. Those are the connectors that the harddrive uses. You have to get one that is compatible with your motherboard / harddrive controller. With newer computers its usually seriel, that's faster, but there's occasionally still IDE / PATA compatible boards made. OEM is what a company would buy if it was building a computer to sell. You can get them yourself, too, sure. There's a few differences though: There's no partition table made, though this is sometimes true of retail, you KNOW you'll have to create one. There's no warranty. (as the reseller would normally provide this) There's no fancy box. ---- oh yes, the above answer pointed out which I didn't that OEM wouldn't come with cables, but the most important thing is the _no warranty_ part. If you buy Western Digital, for example, definitely do retail. --- Nicki: your link refers to consumer products, which includes retail harddrives, not OEM.