For the boot drive, it shouldn’t matter which version of Mac OS X it’s for (assuming 10.7, 10.8, 10.9, 10.10, 10.12, 10.13, 10.14, etc at least), the only requirement is that it has Disk Utility – which they all do. For the former situation where the boot drive is being modified, the easiest solution is to boot from another drive and run Disk Utility from there instead. Once a removable disk has been unmounted, it can safely be disconnected from the computer.Typically the “Couldn’t Unmount Disk” error pops up when the currently boot drive is being modified, or if a disk was trying to be erased you may find the erase failed with a couldn’t unmount disk error. In Mac OS X, select the disk on the desktop and either drag the disk to the trash (which changes to an Eject icon), or select 'File→Eject' from the Finder's menu bar. In order to unmount a disk in Windows, open 'My Computer,' select the disk, and click the 'Eject this disk' option in the left sidebar. Several types of disks can be unmounted, including external hard drives, USB flash drives, iPods, flash memory cards, and disk images.

Therefore, before removing an external data storage device, such as a USB flash drive, the disk should be unmounted to avoid possible data corruption. Since unmounting a disk prevents the computer from accessing it, there is no risk of the disk being disconnected in the middle of a data transfer. When a disk is mounted, it is active and the computer can access its contents.

Of course, in order for a disk to be unmounted, it must first be mounted. Unmounting a disk makes it inaccessible by the computer.

Could not unmount disk usb