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Select your external storage device in Disk Utility. If your storage device isn’t using a GUID partition map, you can create one with Disk Utility. Make sure that the Partition Map Type shown for your storage device is GPT (GUID Partition Table).

Select Storage from the sidebar, then select your storage device from the list of volumes. Here’s how to check:Ĭonnect your external storage device to your Mac.Ĭhoose Apple menu () > About This Mac, then click the System Report button. And to function as a startup disk, it needs to be using a GUID partition map.

First, make sure that your storage device is properly formatted.įor best results, your external hard drive, thumb drive, SDHC or SDXC card, or other storage device should be formatted as Mac OS Extended, not FAT, ExFAT, or NTFS. If you install the Mac operating system on an external hard drive, thumb drive, or other storage device, your Mac can start up from that device instead of your built-in startup disk.