Select Erase and then type in a name for the external hard drive.Click on the name of the external drive in the left panel of the Disk Utility.Open the Disk Utility tool on your Mac.Start the computer and plug in the external HDD.To format an external hard drive on your Mac, you need to complete the following steps: How to format external hard drive for Mac Follow the instructions bellow to format your external hard drive to exFAT. Your mac should then be able to read the hard drive even if you can’t upload or edit files. To get around this problem you will need to revert to NTFS formatting using your windows PC. This can happen if the external hard drive was formatted with a windows computer. You may encounter that your Mac with M1 chip cannot mount the external hard drive event when using exFAT formatting. Mac cannot mount external drive even with exFAT formatting. How to format external hard drive for Mac.Mac cannot mount external drive even with exFAT formatting.