You have just purchased a software for your Mac system but it can be installed because of dmg file No mountable file system error. This is quite a serious nad frustrating situation as the software costs so much and now it is of now use. As you know all the softwares for Mac comes as dmg file which is actually a disk image. You have to open the file in order to install the software. But sometimes it does not open and shows dmg file No mountable file system error. You will thing that the dmg file is corrupted which is actually the cause in most cases, you must get another fresh copy of it if possible but in some cases this particular dmg file No mountable file system error shows up with multiple dmg file. Then it must be an issue with the Mac Operating System. You can fix dmg file No mountable file system error by reinstalling the Mac OS but in the process you can lose some important data. You do not have to worry about it as it can be recovered using Time machine or your personal backup. But if the backup is not there then you can use Mac data recovery software. It is capable of restoring all your data in the Mac system.