Stupidity, mostly.
Fantom has had the 2tb models for about 4-5 years now. It started off GD2000, then the model numbers changed as the product was slightly renamed (GDP2000EUS, GD2000EU, etc) and new variations added (g-force, green, quad, etc).
There are two main drives -- G-Force (7200rpm) and Green (5400rpm).
Both can be black or silver, and the green drives has a green oak tree branch around the FD logo on the side of the drive.
'Quad' is just a Mac drive, with Firewire 400+800, in addition to USB2 and eSATA.
I'll be honest with you -- I see no difference between the "green" and "non-green" drives. Speeds are pretty much identical, even with eSATA (SATA I).
But because the model numbers differ, some companies (
Amazon and
Newegg mostly) list them as separate drives. They're not. So you get this silly price difference on the same drives. However, when the drives are shipped, they're usually not the exact number shown. I purposely wanted an older model for an old and cranky eSATA card, but got the new model. I ended up just replacing the old VIA eSATA card instead.