You should identify your Mac model and check which macOS/OS X can your Mac run. Using different USB ports also affects the speed. You should make sure the USB is compatible with your Mac's ports so that your Mac can successfully detect the USB flash drive. You can back up the USB or transfer important files and then format USB on Mac to get a clean flash drive. The size of the macOS installer has become larger and larger, the installer of Monterey is 12GB, and that of Big Sur is 13GB. Theoretically, an external hard drive such as SSD and HDD can also be used as a bootable macOS installer, but a USB flash drive is always recommended given that it will be quicker compared with a standard hard drive. A clean flash drive with 15GB at least.You need a bootable Mac to create a bootable macOS installer. In addition to using the bootable macOS installer to troubleshoot a malfunctioned Mac, creating a bootable installer for Mac also helps when you need to:

When the MacBook Pro won't boot from internal hard drive, you can use a bootable macOS installer to boot Mac from USB to bring the Mac back on track. Mac beats its competitors for delivering excellent performance and being user-friendly, but inevitably, a machine even your well-equipped Mac could go wrong from time to time such as MacBook won't turn on.

When do you need a bootable macOS installer Or, you can use a Rufus for Mac tool to help you create a bootable USB drive for macOS. If so, this post is right for you on how to create a bootable installer for macOS.

But sometimes, you may need a bootable installer for Mac. Usually, Mac users straightforwardly upgrade macOS on their Macs. It is used to install an operating system on your Mac computer. Generally speaking, a bootable installer for macOS is an external storage media with a version of macOS installed. Essential preparations before creating a bootable installer

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