I had this same problem too. The Blu-ray drive was reading discs just fine in the XMB, playing them flawlessly and installing data files onto the system without issue, but Multiman just couldn't detect discs or, when it occasionally did, would fail in the transfer and throw off data read errors.
The simple answer was laser lens cleaning. I popped one of those laser lens cleaning discs into the drive and ran it a couple of times and now Multiman has no problems detecting discs. I find that it works best if I open Multiman with an empty drive and then add discs while in Multiman. If Multiman chooses to be churlish, I just restart the system again with an empty drive and pop in a disc after booting Multiman, and it works fine.