When you select a brake disk you have to look after several factors. Especially then when it is a backfitting disk from a foreign manufacturer.

• Affiliation:
  Currently there are two ways to fix a brake disks on a hub. On the one hand the 6-hole affilation after IS2000 (International Standard) and on the other hand the center-lock affiliation which got invented by Shimano. As the name says the disk gets fixed on the hub via six spikes in the 6-hole affiliation. In the center-lock affiliation hub and brake disk have a gearing. The brake disk is clipped on and fixed via a central nut.
  You can convert center-lock hubs to 6-hole with an adapter so that you can optionally use both disk types. But it doesn't work conversely.
  Older forms of fixation with three or four boreholes as well as the 4-holes affiliation of the Rohloff Speedhub are the exception. Furthermore the first generation of the Shimano Saint group has a bigger center-lock which has been adjusted.
  
  
• Diameter:
  The diameter of a disk brake should always accord to the current disk. If you want more brake power you can use a bigger disk. Thereby you have to observe that you need an according adapter for the caliper to adapt it on the bigger disk. Furthermore the approval of the fork or rather frame manufacturer concerning the size of the disk has to be observed.
  
  
• Height of the brake disk:
  This refers to the hight of the surface on which you finally brake. In no case this height should be smaller than the brake covering. The height of the brake disk can't be easily determined when it comes to modern disks with a wave profile. Then you have to stick to the recommendation of the manufacturer.
  
  
• Material:
  Disk brakes are generally made of steel. But in some leightweight construction models aluminium is used. Then you absolutely have too look after the use of suitable (and specified by the manufacturer) brake coverings. Because when you select a wrong brake covering this can lead to a collapse of the brakes or a downfall at worst!
  
  
• Maintenance:
  Disks with a slight wobble can be regulated with suitable tool or with parallel nippers. This requires a little manual skill. If brake disks are strongly deformed or scratched by contact with stones they have to be changed as well as in the case of excessive wear.
  Disk brakes can be cleaned with isopropanol or with a brake cleaner.
  A blue coloration of the disk is a sign of a thermal overload of the disc. In this case you should obstruct a bigger disk and/or adjust your braking behaviour.