Unable to access photos on external hard drive after upgrade to Big Sur

OK - you are looking at an empty library.

This is probably not the library on your external drive. It is probably the default empty library in your pictures folder. You can check by opening preferences which will tell you where the open library is located:

Hold down the option key while starting photos, and choose the library that is on your external drive to open. If you need it to sync with iCloud, then open preferences, and click the "use as system library" button.

GSWA wrote:

.... There are no annual albums for example. I hope there is a solution otherwise I will need to wipe the whole system and reinstate Sierra etc

Are you looking for the Moments, Months, Years in the Photos view?

The "Moments" have been renamed to "Days" and you should be seeing them in the "library" section in the sidebar. Can you confirm, that the sidebar in your Photos window is looking like described here?

Starting with a "Library" item and not with "Memories"? If yous sidebar is starting with "Memories" you are a victim of a bug, that is typical for some older Macs with an older graphic card model.

The first item "Library" is giving us access to the main library views. And it will not be shown if there is a problem with the graphics. I can reproduce this bug, when I am starting a Mac in Safe Mode and then launch Photos. The Photos.app is limited in this mode, because the advanced features of the graphics card are disabled. I have not seen a fix for this problem, other than ensuring that the Mac is running in normal mode and not in Safe Mode.

From Sierra to Big Sur is a rather large step and Photos versions differ.

Also make sure the external disk is formatted as MacOS Extended or APFS. Non-Mac formats like FAT are not supported for Photos libraries.

The problem with incompatible volumes is they can work - until they don't. Operating the library on one can result in a gradual build up of errors in the library until it eventually stops opening, otherwise malfunctions, or a later OS version decides it can't work with it. To be honest it is ridiculous that the app doesn’t detect an incompatible volume, and refuse to work with it (with appropriate message) until you sort it out

Again - what is the format of the storage your library is on? If it is incompatible with Photos, then all bets are off with how it will perform.

OK - that should be fine. Do you use iCloud photos? If so, it is possible there is still a resync going on. What does the sync status say at the bottom of the all photos view?

Disks can be reformatted with Disk Utility but doing so will erase them. So back up first. Is it necessary? What format is the disk?