Make External Disk Writeable

SantaSadler wrote:

This is confounding for me. I've been updating the volume periodically for a long time. I'm at 12.7.6. Maybe the last update "fixed" something. If I erase the volume will I be able to write to it then?

If that is an external macOS boot drive, then you will need to navigate to the "Data" volume in order to write to the drive. Preferably you will want to write to the "Users/Shared" folder on that external drive's "Data" volume since you may not have permissions to access the macOS home user folder(s) on that external drive.

It is possible that the "Data" volume for that external macOS boot drive is not currently mounted, so you may first need to manually mount that "Data" volume located on the external drive using Disk Utility.

If you really want to save data to that external drive, then you really should create a new APFS volume on that external drive which both your currently booted OS can access....plus when you boot the external drive it will also be able to access that new APFS volume. Just make sure to select "Ignore Ownership" on that new APFS volume so you don't encounter permissions issues between accessing files on that new APFS volume.

Add, delete, or erase APFS volumes in Disk Utility on Mac - Apple Support

FYI, it is very risky trying to save data to another macOS boot volume. You will end up with a huge mess in regards to knowing where all your files are located, especially if the contents of those files change (especially when one copy exists on one boot volume and another copy exists on the other boot volume). Just don't do it. At the very least utilize a new APFS volume so all files are easily located & accessed.

If Time Machine is backing up to that drive, you can't.

If the volume is formatted with the Windows NT Filesystem, reformat it or install an NTFS for Mac product.

If neither of the above applies, use the Disk Utility's First Aid function on it; this may also happen if the drive is locked in hardware or has sensed that it's about to fail.

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SantaSadler wrote:

Thank you. This helped me find and overcome the issue. I think it was manually mounting the Data volume that did the trick. In spite of mounting it the warning message remains in the Get Info window. And there is nothing being displayed that I could see that disk writing is failing due to the unmounted volume.

Case closed, thank you again

The "OSX1267" volume is mounted, just not the "Data" volume which is magically combined behind the scenes to make everything look like a single volume to mimic the old "Macintosh HD" single volume. If you look at the "OX1267" volume while the "Data" volume is not mounted, you will still see a "Users" folder, but you will see it is empty. Only after mounting the "Data" volume will that "/Volumes/OSX1267/Users" folder contain subfolders for the macOS user accounts used by that external macOS boot drive.

FYI, even if you can write to the root of that "OSX1267" volume when it is connected as an external data drive, you will end up corrupting the system volume where it won't allow the external "OSX1267" system to boot due to a corrupted system volume.

Unfortunately Apple's attempt at keeping things looking the same & trying to "simplify" the experience does not really simplify the experience when dealing with multiple macOS boot volumes (this is not something Apple really considered).

The top level of a macOS boot drive can't be written to manually.

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