I think of the disc as the "stump jumper" and the cross section bolted to the spindle as the rotor.
I have had one work loose as yours did. I reinstalled it and made sure the bolt, and spindle threads were clean and also used blue locktight. Look up the bolt size and hardness on the internet for torque specifications.
IF after you have installed the rotor back on the splined spindle, the whole disk can be rocked back and fourth, either your lower spindle bearings are shot or more likely the C clip holding the bearing in the bottom of the gear box has deformed, making the bearing (which is a roller bearing with a tapered race designed for shimming to specs) loose.
The fix is to pull the rotor, (be careful here, once bolted on tight, it will not come off easy. Rigged HD gear puller is the only way I get mine off. And I do it with the mower standing on its nose, suspended from a heavy duty chain fall, leaning AWAY from me.) pull the oil seal on the underside of the gear box and look at the C clip. If it is deformed or out of the groove, it will be obvious. One symptom that things have gotten loosey goosey is that the oil seal will fail and the gear box will have drained out.
