WD Sata drive connector mismatch

I purchased a "Western Digital Caviar Black WD1001FALS 1TB 7200 RPM 32MB Cache SATA 3.0Gb/s Hard Drive - OEM" from Newegg for my new xps 420.

One of the cables (smaller one) from the 420 does not fit the drive connector.  Are they incompatible? Is there a converter?

More detailed description:

I'm not sure if it's the power or data cable. I thought the data cable had a long thin profile, and the power cable shorter, but the diagram in the Dell manual is tiny and seems to point to the squat cable as Data, ad the thin, flat one as Power.

The long, flat connector/receptacle looks more like a series of little flat rectangular fins that are matched up when connected.

It's the narrower, squat, connector that does not fit. The disk has a slot with receptacles for eight pins in two rows (4x2). The cable from Dell also has 4x2 pin config, but the plastic around the 4x2 pins is larger than the space surrounding the receptacle, so it can not fit in. Can't be squeezed in either.

Both the primary drive that came with the Dell, and the WD have the same 4x2 slot, as well as a smaller version of the narrow flat receptacle right next to the larger one. But the samller (power?) cable for the primary fits the narrow flat slot. So the primary and secondary have different power connectors, if that makes sense?

Thanks,

PeterK