True Image of course needs to be able to write to and read from disc for volume image recovery but its "rescue" incarnation (self-contained Linux boot) is apparently not current with the standard which has progressed over the last decade.
So, an image can be made to a HDD (for quickness) and then the user writes that to a disc (for additional redundancy) but if it becomes necessary to restore directly from the disc, True Image would not necessarily be capable of reading it.