14" disc media problem?

I'm hoping someone can tell me something I'm missing here, cause I just can't believe what I'm seeing.... HP7900A disc drive, using the standard media for it. The media for this drive is virtually identical to an RK05 cartridge. I have 4 pieces of media. I pulled a cartridge from the stack and tried to spin up the drive. Blowers come on, but when you hit the load switch the spindle just sits there. After three days of checking all the sensors, tracing the wires, pouring over the schematics, still no go. So finally on a whim (yeah, it's the first thing I should have tried) I pulled out the cartridge, popped the cover on the drive and defeated all the sensors. The drive spindle motor started like a champ. Put the cartridge back in - no spindle motor. However, I noticed that when the cartridge is in, the relay that gives AC to the spindle motor does engage correctly, but no spindle movement. So to make a long story short, I tried the other three pieces of media I have. I put in each piece of media at least 20 times - every time the spindle starts up fine. But, put in that first piece of media, the relay engages, but the spindle sits still. What gives here? What on earth could be wrong with a media cartridge that would cause the spindle not to move? I tried lifting up the platter slightly and turning it, it seems to turn just as freely in the "bad" cartridge as in the other three. I'm just really confused - is there something I'm missing here??? Dazed and confused.... Jay West