No Mountable File Systems error can bother any one using Mac any time you are trying to open or mount DMG file you can encounter this error. This error will not let you install any software as the .dmg file will not load. DMG file stands for (Disk mounted graphics) files these are like the real disks we can compare it to any of the CD, DVD or the hard drive. If you have to access any information from the DMG files you need to firstly mount them.
Till now you won't face any problem you will mount the DMG file and access the data but the problem area comes now. Mac OS X is capable of creating an image of the data and keeping a back up for the data. But when you will try to access the data from the image created, like the disk on the system the dmg file swill mount and there after you will be able to access the data but some time the dmg files refuse to mount and that's the time you face the error No Mountable File System.
This problem occurs due to the corruption of the file system, ROBOT may fix the problem. If the problem fixes its really fine but if it doesn't then you can mount the files from the command line –hdiuti, this command is used to manage disks. You need to run this command on the command line as root.
$ sudo hdituil attach file name.dmg
Even then if the data on the Mac hard drive is not accessible it might happen that the data is damaged in such a case a mac utility software can hp you by this you can even manage partition of the Mac volume as well as the mac recovery software will help you recover data from the corrupt Mac volume.
For more understanding on the file systems particularly HFS (Hierarchical File System) file consult experts Wikipedia on Hierarchical File System system
