macOS
Shine a spotlight on the solutions to your most irritating Mac problems
> Mix multiple APFS versions safely
My Mac normally starts up in Sonoma on an external SSD, but has Mojave installed on its internal drive. Why I do see warnings about the external drive being incompatible? Would reformatting and reinstalling them help?
These arise because those two versions of macOS use very different types of the APFS file system. Although that supplied with Sonoma (APFS 2235 and 2236) understands the version in Mojave (APFS 945), the reverse isn’t true, as Mojave can’t understand the boot volume group and Signed System Volume in Sonoma. So every time you start your Mac up from its internal drive, if the external SSD is connected, you can expect it to warn you that it’s incompatible.