Question Accidental installation of Mojave on external HDD. How do I re-format the HDD?


Mar 16, 2011
Hi all,

I'll preface this by saying that I'm a PC user and know nothing about macs. girlfriend thought she could save the Mojave update on her external HDD (which, by the way, is used to store all of her pictures, videos, work, etc.), and install it on her macbook at a later time. Unfortunately, the update installed on the external HDD, which is now functioning as a boot drive. If she boots with the HDD, she can still access those files. I think I can pull her data off that drive with an external HDD of my own (appropriately formatted, exFAT will work I think?), however where we're stuck is formatting her external HDD afterwards.

How would we go about doing this? We've tried plugging it into my PC and looking for it but didn't find it (unsurprisingly), however she can't even see the external drive if she boots off of her macbook drive. Is there a mac equivalent of a post screen and bios that I can format the external HDD from?

Last edited:


Jan 15, 2018
Unplug all external drives from the Mac and update the Mac to Mojave, or highest possible update. Then once you are updated, plug in the external. You then should be able to transfer your files. Highly encourage full back ups prior to making any changes. Also, even with the external plugged in, under preferences, you can choose which start up drive you would like to use.

As a note, you should be able to simply start your computer with no drives attached and then attach the external and transfer your files.
Which version of macOS is the internal boot drive running? If Sierra or earlier it will not be able to see the external if it was formatted with APFS (High Sierra and later). The solution is to update the boot drive to Mojave.

If you cannot see the drive in Disk Utility. Click View > Show All Devices. [rant]I don't recall when Apple started this horse pucky of only showing recognized partitions by default rather than the full drive. MacOS is supposed to be intuitive and this is a step backwards. Disk Utility used to be much easier to use.[/rant]

If you can't see a drive in Windows. It's because Windows does not recognize Mac drives. Alll you'd be able to do is format it anyways without third party software to read/write Mac volumes. Even with formatting you'd have to use the command prompt. Because for some reason MS decided to make it difficult to format GUID partitions. Anyways you'd at least be able to see an unrecoginized partition exists in Control Panel > Administrative Tools > Computer Management > Storage > Disk Management.

For future reference. For external non-boot volumes (especially in a multi-OS environment). I would recommend formatting using ExFAT. As it is a file system supported nearly universally by all OSes. Except really old ones and possibly a handful of obscure ones. Unless the external is being used for Time Machine backups.

You can also select the boot drive at the time of boot by holding down the option key before the startup chime. I believe new Macs did away with the chime. So, as soon as you turn it on.
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