Formatting My External Drive

What is the make & model of the external drive & how is it connected to your Mac?

What do you mean by "it fails." Do you mean you cannot format it or do you mean that after you have formatted & installed macOS it won't boot?

BTW, if you are running High Sierra and it's a traditional hard drive (not an SSD) you should format it as Mac OS Extended Journaled. Not APFS.

Attempting to use your external HD as a boot drive will be the definition of SLOW. Why do you want to do this?? Also, MartinR is correct about the format the External Hard Disk needs to be in order to be used as a startup drive.

Run DriveDX on the External Hard Disk and see if it is failing. WD Passports are very low quality drives and tend to overheat due to poor cooling. If DriveDX shows any errors then my hunch of a failing drive is correct.

rage37 wrote:

I’m using ExFAT

It is a WD My Passport Model number: WDBPKJ0040BBK-WESN

And is connected by Micro B to usb A

Also when I say it fails, I mean when I format it and get ready to erase when I click continue it fails when formatting.

OK, let's take this from the top.

In your initial post you said "format it to boot master." Are you trying to set the partition scheme to Master Boot Record (MBR) and then formatting the disk as ExFAT? If this is what you are attempting, it won't work. MBR partition scheme only supports disks up to 2TB. You said your drive is 4TB.

If you set the partition scheme to GUID and then format as ExFAT it should work. GUID has no practical limit on the maximum disk size.

If you are trying to create a bootable external drive for your Mac, it must be set to GUID partition scheme and, since your WD Passport is a traditional hard drive and you are using High Sierra, you must format it as Mac OS Extended Journaled.

If this is not what you are trying to do, let us know the particulars.

rage37 wrote:

It just freezes when I try to boot from it

1 - After you set the GUID partition, erased & formatted the drive, did you install macOS on it? If so, what version of macOS?

2 - If you successfully installed macOS on the external drive, exactly what procedure are you following to try to boot from the external drive?

To boot for Mac go to Disk Utilities select the drive, select erase and set the parameters like this: Then continue to erase.

You did not note what OS version you are running but the mechanical drive should probably be formatted to Mac OS Extended (journaled) or HFS+.

IF the drive is formatted as ExFAT that is not bootable for Mac OS.

How did you install the OS on it?