Unsupported Photo Library
I've connected external to update my photo library on Big Sur, and now getting unsupported photo library when I try to open it.
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Can you tell us more about the library on the external? Which system version is running on your Mac? Is it Big Sur? Has the library been used on a different Mac with a different system version?
Photos cannot open a library, that has been opened by Photos on a Mac with a newer system version than is running on the Mac where you want to use the library. You can only open libraries that have been updated to the same system version or an older system version.
If the library is on a external drive, the external has to have been prepared as described here: Move your Photos library to save space on your Mac - Apple Support