{"id":47199,"date":"2025-03-25T14:26:30","date_gmt":"2025-03-25T18:26:30","guid":{"rendered":"https:\/\/mjtsai.com\/blog\/?p=47199"},"modified":"2026-02-02T10:21:46","modified_gmt":"2026-02-02T15:21:46","slug":"error-702-installing-macos-on-an-external-drive","status":"publish","type":"post","link":"https:\/\/mjtsai.com\/blog\/2025\/03\/25\/error-702-installing-macos-on-an-external-drive\/","title":{"rendered":"Error 702 Installing macOS on an External Drive"},"content":{"rendered":"

My Intel MacBook Pro died, and even though I have an M1 MacBookAir<\/a> ready to replace it, it’s taken several days to get back to the point where I can run older versions of macOS for testing my apps. I had all the old versions installed in separate partitions on an external SSD. Prior to Apple Silicon, I could just plug this drive into a new Mac and continue as if nothing had happened. But Apple Silicon Macs can’t use Intel boot drives, so that won’t work with the new test Mac. Although externally booting Apple Silicon Macs has been a bit dodgy<\/a> in the past, I figured that 4+ years<\/a> into the transition the issues had mostly been ironed out. My M1 test Mac already had an external SSD set up with partitions for testing different versions of macOS Sequoia. I figured I could just reformat the other SSD and install fresh copies of all the old versions on it. I eventually succeeded, but the process was anything but straightforward and I’m left feeling like Jeff Johnson<\/a>:<\/p>\n

I don’t understand Macs anymore.<\/p><\/blockquote>\n

I was already aware of this limitation<\/a>:<\/p>\n

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There are further complications to this. For instance, an older macOS Installer app can’t be run in a newer major version of macOS. The workaround for that is to create a bootable installer volume, and boot from that to run its older installer.<\/p>\n<\/blockquote>\n

so I created a bootable installer with DropDMG<\/a>. The main problem I ran into is that, after rebooting and running the installer, a good way into the process it would fail with this error:<\/p>\n\n

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com.apple.OSinstallerSetup.error error 702<\/p>\n<\/blockquote>\n\n

The detailed error log showed a problem<\/a> mounting<\/a> the InstallESDDmg.pkg<\/tt> file: “Couldn't mount dmg! (error code 12).” I searched for the 702 error code and various text from the error log and came up with nothing useful. There are<\/a> a<\/a> bunch<\/a> of<\/a> pages<\/a> with<\/a> generic<\/a> advice<\/a> that didn’t apply to me. I’m not using MDM, the Internet connection is working, the clock<\/a> is correct, there’s plenty of disk space, the installer isn’t damaged, M1 Macs don’t have NVRAM<\/a> to zap, Find My is disabled, etc.<\/p>\n\n

I tried multiple installers from different major and minor versions of macOS and multiple different destination drives. I tried running the installer with two different Macs, reformatting both the source and destination drives, and repartitioning the destination to have a single partition\/container. I kept getting error 702. Both of these Macs had previously successfully created and booted from external drives. What was different now?<\/p>\n\n

I found a promising comment on Howard Oakley<\/a>’s site:<\/p>\n

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I tried to install onto an external plugged into the DFU and got a message “The operation couldn’t be completed.(com.apple.OSinstallerSetup.error error 702.)”<\/p>\n

I suspect Apple have made a recent change to firmware\/hidden volumes which requires the non-DFU port.<\/p>\n<\/blockquote>\n\n

This reminded me of one of his earlier posts<\/a>:<\/p>\n

On each Intel Mac with a T2 chip, or Apple silicon Mac, one of its USB-C+Thunderbolt 3 ports is designated for use in DFU mode. That port isn’t labelled, and System Information doesn’t tell you which is the DFU port on that Mac. Instead, Apple lists them for each model here<\/a>. Beware when reading that support article, as it isn’t internally consistent: for instance, it shows the DFU port as being that on the left of the left side of a MacBook Pro, but states in the text that on a MacBook Pro 14-inch 2024 with an M4 chip, the DFU port is that on the right of the left side instead.<\/p>

DFU mode is seldom required in T2 or Apple silicon Macs, as it’s how you can refresh or restore the firmware in them. But if you do need to use it, you’ll need to connect a USB-C (not Thunderbolt) cable to the DFU port and perform the procedure from another Mac connected to the other end. However, there is another more common situation in which you need to know the DFU port on an Apple silicon Mac: that’s when you want to create an external bootable disk.<\/p>\n

[…]<\/p>\n

If the external drive you’re trying to install macOS onto is connected to that Apple silicon Mac’s DFU port, then the procedure is almost certainly doomed to fail. The solution is to connect that external drive to a different port, where it’s likely to succeed.<\/p><\/blockquote>\n\n

Apple<\/a> finally documented this:<\/p>\n

If you’re using a Mac with Apple silicon<\/a>, plug your storage device into any compatible port except the DFU port. Learn how to identify the DFU port<\/a>. After macOS installation is complete, you can connect your storage device to any compatible port, including the DFU port.<\/p><\/blockquote>\n\n

Howard Oakley<\/a>:<\/p>\n

For example, on my Mac mini M4, that’s either the left or right Thunderbolt port, as the middle one is its DFU port. On all other Apple silicon Mac minis, that’s either the centre or right port as you look from the rear, as their DFU port is the one on the left.<\/p>

[…]<\/p>

Reformat that disk as you want to use it, with at least one APFS container containing a single APFS volume in regular APFS format (not encrypted).<\/p><\/blockquote>\n\n

But, alas, that was not the cause of my problem. Several of the failed installations had<\/em> been to drives that were not connected to the DFU port.<\/p>\n\n

I did find one other suggestion from Apple<\/a>:<\/p>\n

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Reviving or restoring firmware might also help if your Mac experiences a persistent macOS installation error not resolved by other solutions for macOS installation errors.<\/p>\n<\/blockquote>\n\n

I was skeptical that reviving<\/em> would help. It seemed unlikely that both<\/em> M1 Macs had bad firmware. Restoring<\/em> an IPSW file for an old version of macOS should<\/em> definitely work, but I didn’t want to erase either Mac’s internal SSD. And I wanted to install multiple<\/em> old versions of macOS externally<\/em>.<\/p>\n\n

But this did give me an idea: could there be something wrong with the recovery partition on the internal SSD? I had<\/em> noticed something unexpected. Apple says<\/a>:<\/p>\n

On a Mac with Apple silicon<\/a>:<\/p>