My reasoning for this order is to limit potential data loss when repairing. If so, then right click on the file, click 'Show Contents Packages'.

I assume that you have downloaded the installation from Mac App Store.

The solution: Use the 'Restore BaseSystem.dmg' method to create the installation USB. RecoveryHD = El Capitan recovery drive.Macintosh HD = El Capitan drive (already installed and working).Sierra = New partition created for Sierra.MSR = Microsoft Reserved (culprit, too?).It could have been due to that, although MSR partition is also known to cause problems in some situations. The disk “Storage” could not be unmounted. (If you disable SIP to use certain third-party software or unsigned kernel extensions, you’re on you own now.) Some Disk Utility stalwarts will also be disappointed that this version omits support for multi-disk or RAID arrays. During El Capitan’s beta testing, one round of release notes from Apple explained the permissions removal from Disk Utility by noting that during software updates, any permission issues would be resolved by an installer, but that otherwise, there was no need. (SIP), also known as rootless mode, prevents modification to these files. But in El Capitan, any system file for which Repair Permissions would have restored these settings can no longer be modified during normal operating. Even so, from all reports, permissions repairs had little real effect for years-it just made us feel better. A printer driver with the wrong switches flipped might not be available to the system or the driver might be unable to access the printer settings file or temporary print queue directories. It made sense that repairing permissions on files for which OS X knew precisely what settings should be in place could fix random faults. Caelenwilson$ sudo umount -f /Volumes/Caelen_HDD WARNING: Improper use of the sudo command could lead to data loss or the deletion of important system files. Upon using the second command, I got an error stating that the disk couldn't be unmounted. You can also burn to a CD, boot it and try to format your external HD from it. See What is the ‘Mac OS X Base System’ disk image on my 2011 Macbook AIr? To get an explanation.

The Mac OS X core system image is not the cause of your problem. Everywhere it really is you get that blunders message, let us know what exhibit screen you're in (Partition, Erase, different), WRITE DOWN the precise words shown and record decrease back. If not 'Mac OS prolonged (journaled)' format it wisely.

Could not unmount disk when formatting