If your Mac system is giving you trouble with the Mac hard drive and couldn't unmount disk then it is really serious matter. The applications that are installed in the Mac system comes as a dmg file which has to be mounted in the volume and then the installing is done. If the disk is not unmounted then you can not mount any other dmg file install different programs. You can use the inbuilt disk utility program to scan for any problem in hard drive and it will try to fix couldn't unmount disk problem. If the utility is unable to fix this then you have to format the drive with the help of disk utility itself. If the any of your files have been lost in the process from the drive then you can get it back by using Time machine. This utility automatically saves the copy of the files of the Mac drive in an external drive which can be restored when required with the help of this application. However sometimes the time machine does not work then in that case you can try Mac data recovery software which can scan the Mac drive for deleted data and then show them in the preview. You can select the files you want and they will restored at given location. This is how easily you can resolve couldn't unmount disk problem. So download the software now.