Mac OS X 10.5 no mountable file system has become a problem for you. It is an error that can occur when you try to mount a dmg file or any external drive to the Mac leopard system. This error will not allow to install any software in the dmg file and if the error shows up with the drive then you won't be able to transfer any file. This may be the problem with particular dmg file or drive so you should check with another drive and file. If Mac OS X no mountable file system error still happens then there must be the issue with the operating system itself. You may have to reinstall the Mac OS X or you can upgrade the operating system to the newer version. This will fix Mac OS X 10.5 no mountable file system issue but in both cases you can lose data from your Mac drive. You can recover the lost data by using the time machine backup but in sometimes it does not work so you should be looking for an automatic recovery tool. Mac data recovery software is the best you can get to restore all the lost files after you fix Mac OS X 10.5 no mountable file system.