The speed is controlled by pin 39 of the interface connector. If this pin is open, or held high when the drive is powered up, the drive will default to 1Gbs. If the pin is held low at powerup, the drive will run at 2Gbs. There should be something that I can control/select the thingy with, but I don't know what or how to.

Low Voltage Differential (LVD), Ultra-2 and Ultra-3 capable drives also support Single Ended (SE) mode. This feature is called "multimode".
Differential is a method of carrying the signals over the cable. Each signal is carried on two wires, with each of the two wires carrying the signal 180 degrees out of phase (meaning, when the signal on one wire goes high, the signal on the other wire goes low). This is decoded at the receiver end by comparing the two together to see what the relative difference is in the two. This is highly resistant to outside interference, as any interference would affect the amplitude of the signal, but not the phase relationship.
I don't really know what SE is but I don't think it's the same as LVD.