After four days, Retrospect had not yet backed up even half of my information - seeming to be quite a bit slower than Time Machine. Several readers pointed out that Time Machine does in fact back up other hard drives, though it seems it doesn't back up anything but Mac OS (journaled). Perhaps this is why the original attempt at using Time Machine failed. I won't know for a while yet. I am currently doing a backup of the two internal drives and will add an external drive when that is done. Two of my external drives aren't journaled and the Time Machine documentation makes it pretty plain this is a problem.
I suspect I will have to copy the files on those disks to drobo separately, reformat the drives and restore the files to them.