Could not unmount disk error is an error message that pops-up when the recent boot drive is being changed. Though Disk Utility functions without any trouble. But to frustrate you could not unmount disk error might come up. This may take place during partition, disk verification and repair and even at the time of formatting. In this case there is no information which clarifies what to do next. As a measure to this situation boot from another drive and run Disk utility. It works on all types of OS X. This will surely help you to fix the problem,regardless of the reason by one of two means. There are other recommended methods also available that will surely fix the error. Firstly,you need any OS X boot drive to finish this job. Attaching the USB boot drive to the Mac and restart. Selecting the attached boot drive,hold down the option key at the time of boot. From boot menu ,opt 'Disk Utility'.Go to “First Aid” and check the disk,then repair if required. If this manual recovery fails then take the resort to recovery tool to get the data back and solve the error could not unmount disk error.. This is fully tested and verified to tackle data recovery due to could not unmount disk error.