How Should I Proceed

[dnanian]

The size of the partitions doesn't matter, but combining drives isn't necessary an easy thing. When you back up with SD, we don't copy the partitions -- we copy the files in the partitions, to a volume whose size can be anything at all (as long as it fits the files, of course).

So, you can do the copy and then manually copy things together, which -- since I assume the files on the other partitions can be Finder-copied -- is probably the best way...

I'd definitely suggest doing the combination on the backup, though. That way you can check it out before committing to it.