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. This pin should have a jumper, or other means of selecting on your backplane or adapter.
Pins 17, 18, and 39 control a device code, specifying the clock rate. At this time, we only monitor pin 39, with the others reserved for future options. Unfortunately, the X15 FC product manual on our website is only listing these pins as "reserved", and the table that defines them is omitted also. This will be corrected in the next revision. If you are interested in this, you can download the product manual for the Cheetah 73 FC, which does have the pins defined, and the table for usage. This would be valid for any of the FC drives that support the 2Gbs clock. Also, double check you have the proper device drivers, as FC is not normally used with Win98.