I don't read the original post as an attempt to import from iPhoto, but an attempt to relocate an existing Photo library.
Regardless, grelber's suggestion would also be mine. Move the old library to the external drive, open Photo and try to import from the new location.
On a Mac since 1984.
Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.
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