< I'm not sure what that means, but I assume that the servo surface < contains a sort of coordinate system that the servo head reads and thus < guides the other heads. If that assumption is correct, why isn't it < possible to realign the thing with another servo platter? First it depends if the servo is embedded or dedicated platter or stepper (or other absolute positioner). For dedicated platter one head is assigned as the servo head and since they are all mechanically one assembly the rest just follow along. Now if a platter is changed it's data may be offset relative to the others. Same thing is true for replacing a head as the RELATIVE alignment between heads is disturbed. This case I believer the disk can be low level formatted and reused but existant data may be lost. If servo information does not exist (DeMIL'd drive) it's likely good for parts as servo tracks are hard to regenerate. If it's embedded servo per head each head as selected will follow the selected track servo signals and making relative head alignment less critical. If the servo is blown there is no way to regenerate easily. For cases of absolute positioners like older ST506/412/225/251 (and a swarm of others) drives getting alignment to cylinder 000 and beteeen heads would be critical for complete data recovery. If the drive is to be reformatted however a bulk erase and reformat will establish new tracks in some cases and make a mess in others. In the case of removable pack drives that is a totally different can of worms though simpler as they so have alignment procedures for field or depot use. Allison