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Many Apple firmware updates in High Sierra and beyond are only available as part of an OS install or OS update and are no longer available as standalone patches. In many cases the updates are required for the iteration of APFS included with a particular macOS version. Without those firmware updates macOS is not able to read the APFS boot volume. These firmware updates are downloaded on the fly from Apple by the macOS installer or macOS update installer based on the serial number of the Mac. For example, restoring a disk image captured from a macOS 10.13.6 APFS reference system onto a Mac currently on and an older version of High Sierra, such as 10.13.3, can result in a non-bootable Mac. This is because the the 10.13.3 Mac does not have the requisite firmware updates to support the filesystem in the 10.13.6 disk image. The only way to get those firmware updates is to update the OS to 10.13.6 at least once.