And the reason is because the connector on the later versions of the Toshiba 1.8-inch hard drives is different.
The older type that works with your iPod goes up to 60GB. There are two physical sizes, base on whether the drive has one platter or two platters, and these correspond to the "thick" and "thin" iPod back panels.
I have a 4th gen iPod with a standard compact flash card for storage, which replaces the hard drive through an adapter. I believe it's this one, or one like it
It fits in the "thin" back panel. The only modification I needed to make was bending over the pair of "jumper" pins that sets it for master or slave (needs to be set for master). Currently, I only have an 8GB CF card in there, but it works great. At some point, I may get a 64GB CF card for it. Solid state storage is much better than a hard drive for everything except sheer capacity.