Definitely it is quite hard time for you whenever partition failed could not unmount disk come across. Typically this type of error appear while partitioning, repairing or verifying disk permission and formatting of disk using disk utility which would prevent you for doing all these tasks. According to experts advice when there is modification done in boot drive which result as partition failed could not unmount disk error. So in order to overcome from this problem one can try to boot the system from the other drive thereafter running disk utility in it. For this you can follow these steps which are mentioned below.
Resolving unmount disk issue from USB drive.
- Firstly make attachment of USB boot drive with Mac and then reboot
- Now option key needs to be hold during boot, choose attached boot-able drive.
- Then from boot menu select Disk Utility.
- Move to the “First Aid”, verify disk and then if required repair it.
- Fix couldn’t unmount error.
Resolving Disk Utility errors through Recovery Partition
- First of all reboot Mac by holding "Option” key down along with choosing recovery partition.
- Secondly choose “Disk Utility” from boot menu.
- Move into “First Aid” for verifying as well as repairing disk, now select “Erase” in order to format disk.
Hopefully, partition failed could not unmount disk error has been resolved by this way. If there is data loss happen restore from the updated backup. In backup unavailability you can consider Mac Data Recovery Software.