Actually, when I last tried to merge 2 partitions in PM, It didnt actually merge them as such, it only made the second partition a folder in the first one, so I still had 2 partitions, just that rather than having 2 Drives, I had the one drive with an extra folder.
The only real way then, that I had, was simply to fill up the partition I wanted to keep, and then resize the partitions, shrinking the one that was to be removed and growing the one to keep to make more room, and move the files over... until eventually all the files had been moved over.
These days, HDs are so cheap that its not worth bothering with, and all my PCs have at least 2 HDs, my 4 main PCs have 4.