Upgraded to Sierra, but can't access Photo library on external drive

[Brother Seamus]

Yesterday I foolishly upgraded my system from Mavericks 10.9 to Sierra 10.12.
Had ben holding out for years on this because I hate how they've destroyed the whole skeuomorphic thing.

Apart from looking awful, and in my opinion, running slower, I now have a big problem.
My iPhoto library is on an external drive.
When opening up "Photos" now for the first time, it asks to identify the library, and then somehow starts trying to import the whole thing onto the Mac hard drive. Then I get a message saying there's not enough space for that.

How can I continue to use the library on the external drive?
Can't open anything at the moment.

Thanks.

 

[Brother Seamus]

Still struggling with "Photo" which will not let me access my library.

When I try to open it, it starts "preparing my library", takes several hours to get to 8% and then gives me this message.

The library currently about 450GB and is on a 1TB drive.