![Disk Disk](https://ugetfix.com/wp-content/uploads/articles/askit/fix-couldnt-unmount-disk-error-on-mac_en.jpg)
Then, input the “diskutil list” line, which will list out all the disks on the Mac. At this time, you ought to find out the disk that you want to force unmount.
My suggestion is pretty much like a broken record: If you're having trouble installing, try creating a bootable USB flashdrive version of the installer. Boot from the flashdrive and try the install that way. I highly recommend the free little app 'Boot Buddy' to create the installer: I suggest a flashdrive 16gb in size. You can -try- 8gb if that's all you have. Boot from the flashdrive installer - Open Disk Utility and NUKE the internal drive - Erase it back to HFS+ with journaling enabled.
Open the installer and do the install.