Fire Drill Wizard may fail to recognize that a volume fits on a disk if the same disk is being used for another volume

When using the Fire Drill Wizard to prepare the fire drill configuration, you can assign disks for the snapshot volumes. If you assign more than one volume to the same disk, the Fire Drill Wizard requires that the disk size be large enough to accommodate the size of both volumes combined, even if one of the volumes is being assigned to another disk as well. For example, if you have a 10-GB volume assigned to disk A and disk B, and a 5-GB volume assigned to disk B, the Fire Drill Wizard only allows this assignment if disk B has at least 15 GB free. (893398)

Workaround: Assign volumes to separate disks or ensure that if more than one volume is assigned to a disk then it is large enough to accommodate all the volumes assigned.