This appears to be a pretty known error. The only problem is, no solution seems to fix it. Lately I have been trying to download a few trials for software such as Little Snitch and Ableton(they both are amazing products from what I have heard). The only issue is for both of them the Disk Mounter gives me an error "No Mountable File Systems". This just started recently so I really have no idea what is going on. I tried opening them with Disk Utility as well and I get the same error. I also tried terminal commands and got the same error. There really seems to be no winning on this one. I really need to fix this ASAP.
I finally tracked down my "no mountable file systems" errors to com.lexar.driver.LexarFilterScheme - this is installed as part of a Lexar Jump Drive's secure II vault driver that my work requires me to use. Doesn't seem to be a newer version on Lexar's site.
As soon as I deleted /System/Library/Extensions/LexarFilterScheme.kext and rebooted, the error went away. Took me quite a while to isolate it to that piece of crap.