Install MacOS Tahoe on external SSD

You also need to erase the whole physical drive so that the partition table is rebuilt as well since sometimes the partitioning from the factory can cause problems (especially when used as a boot drive). See the following Apple article from the Internet Archive since Apple has changed & ruined the most recent version of the article:

https://web.archive.org/web/20250909095655/https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/mac

You are not using the proprietary Samsung software to manage that T7 SSD are you? If you have enabled the T7's builtin hardware encryption, then that can be a problem as well.

macdan67 wrote:

I tried with another external SSD drive: the problem is the same. Tahoe won't install. For me, it only works with a normal 2.5-inch external drive (not SSD)! Any ideas for the SSD? Thank you.

Did you try erasing the whole physical drive on the T7? This is critical to try.

Did you enable the T7's built-in hardware encryption? If so, then it cannot be used as a boot drive because you won't be able to unlock the SSD. If you did enable it, then disable the built-in hardware encryption.

I would try using another USB cable....try using a good brand high quality USB cable. A high quality cable is very critical especially when using an external USB boot drive. A lot more data is being transferred in both directions across the USB cable than when you are using that same drive for a data only drive (or a backup drive).

Connect the T7 directly to the computer and disconnect all other external devices in case one of them is causing a problem.

Plus Macs can be very picky about the drives used for booting. Some drives just cannot be used as a boot drive.

macdan67 wrote:

Thank you! But I'd prefer not to touch the internal drive...

On an M-series Mac, It is no more risky than installing macOS to an external drive. Installing macOS to an external drive may still modify items on the hidden APFS system containers since they are used to boot from any drive (internal & external).

The option proposed by @Ian does require having at least 80-100GB of Free storage space on the internal SSD just to install a bare OS (you would need even more space if you wanted to migrate the items from the existing installation...not recommended unless you have very little data to migrate). Ignore the very misleading "Available" storage value since it is not synonymous with Free on macOS.

This option would provide important information about the issue and eliminate any possible USB & external drive related issues. If the same issue occurs installing into a new APFS volume on the internal SSD, then it implies you will likely need to even reinstall the existing OS on the internal SSD.

Finally, I think there's another problem. I tried again with a standard external drive (USB-C, not SSD), but that doesn't work either! When I try to continue the installation, I can no longer select the external drive. I get the following message: this volume cannot be used because it has a secure token without a user.

It's the same with both SSD drives I tried.

Any other ideas to avoid this message?

You may have some sort of issue with the Mac's secure enclave. Maybe a DFU Firmware Revive should be tried as it resets the secure enclave chip. Or it may require a more drastic measure of a DFU Firmware Restore which resets everything and destroys all data on the internal SSD.

How to revive or restore Mac firmware - Apple Support

Unfortunately Apple doesn't provide much information about the secure enclave chip, the new concept of "Ownership", or secure tokens. Unless someone who has this problem actually posts detailed information about the issue along with a solution, we are just grasping at straws.

I think we are reaching the end of what can be accomplished remotely online. I think you may need a local Apple expert to actually see your system in order to notice any small nuances that may be a clue to the problem. Some issues just cannot be solved remotely.

macdan67 wrote:

Thank you for all these ideas. I just tried again with a Crucial X8 SSD. I have never enabled a hardware encryption on this device...

Thanks for confirming.

First, I erased the whole physical drive.

I have selected the correct USB port (not DFU). On MacBook Pro M4, the DFU port is on the right side...

If you are looking at the Magsafe side of the laptop, then yes....the DFU port is the right-most port, but I would not say it is on the right side (implies the actual right side when using the laptop normally). The DFU port is the front left USB-C port. I really dislike how Apple identifies the DFU port & it would be nice if Apple would just standardize the location.

As you suggested, I tried another USB-C cable (from Apple).

No other device was connected.

Is this USB-C cable from Apple one which has a lightning bolt icon on the connector? If not, then you are using the wrong cable since it is only a USB-C charging cable with support only for USB2 transfers.

You need to use a USB-C cable which has either a lightning bolt icon on it, or a USB symbol or USB designation on it indicating the supported USB3 protocol it supports.

Do you know of a compatible external SSD drive model for booting?

Thank you for your help!

I don't purchase any pre-made external drives as I prefer to build mine from parts I know & trust. An OWC Mercury SSD should work though as OWC caters to supporting Mac hardware.

With an M-series Mac you must make sure not to connect the external drive to the Mac's DFU port during the install process. See the following Apple article for details:

How to use an external storage device as a Mac startup disk - Apple Support

FYI, the DFU Port is not at a standardized location, so you must review the Apple document to confirm which USB-C port is the DFUI Port (way too hard to have a standard on devices produced by a single tech company).

Just as an aside, it is not necessary to use a separate SSD to install test operating systems.

You can simply create new Volume in seconds on your internal SSD using Disk Utility and install macOS on that.

It is not the same as a fixed partition because the size of the volume is "elastic" . . . it will stretch or shrink to accommodate whatever you put on it.

If you ever find you don't require it anymore you just delete it.

I know you have stated that you have formatted the SSD correctly but have you?

It is essential that when you format the SSD you select the correct part of the disk which will be verified by the words "Physical Disk".

If you don't do that you will never be able to install the OS.

You also need to use the correct port at the back as someone else detailed.

Why would you prefer not to touch the internal drive? Creating a volume is both easy and safe.

macdan67 wrote:

Thank you! I formatted the drive as you instructed. And I chose the right port.

FYI, probably better to say "correct" port unless you actually mean the port which is physically located on the right. They make comedy routines about these dual meanings/associations ;-)

I also tried installing Sequoia on the external drive first, thinking I would upgrade to Tahoe later. But I couldn't install Sequoia either! I even called Apple support: I was on the phone with them for an hour, but they couldn't give me a solution.

Sometimes it is best just to go to an Apple Store or an AASP for assistance since some things just cannot be resolved remotely.

Well, they did give me one: see if anyone I know has a recent Mac with Tahoe and try my external drive on their computer...

That would work for a bootable macOS USB installer, but not a full blown macOS installation on an external drive. On M-series Macs, a bootable macOS external drive is tied to the computer it was created. If you try to connect a bootable macOS external drive to any other Mac, it will want you to reinstall macOS on it.

Unfortunately Apple agents don't necessarily have any real world experience with support Macs....they rely on Apple's support scripts & sometimes very incomplete & misleading documentation. Having a local Apple expert physically examine the Mac & your T7 SSD and assisting you is probably your best option right now. Someone with physical access will be able to notice the little things which may provide the clue necessary to figuring out the problem.

I think the only solution will be to try on my internal drive...

It seems impossible for me to install an OS on an external drive from my MacBook Pro M4...

Or take it to an Apple Store or an AASP for assistance. Sometimes having physical access to all devices is necessary since an experienced tech will notice the very little things which tend to provide the valuable clue, but these important little things can go unnoticed by the user.

How are you trying to install macOS on the external drive? There's a tutorial here:

https://appleinsider.com/inside/macos-tahoe/tips/how-to-make-an-external-bootable-drive-in-macos-tahoe