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.
