Photos app won't let me import photos.

tdresscher wrote: …We got a new, bigger one for in this case (Lacie 4 TB). I copied the old photos library to it,

You didn't tell us what macOS the M3 MacAir is using-- can you let us know? Trying with the internal Library was a good test, so let's start there for some more information: how is your external drive formatted? To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. The drive must be connected directly to the Mac by cable, not networked, clouded, NASed, etc. Additionally, the drive can not have had Time Machine on it since it was formatted. See this:

Move your Photos library to save space on your Mac - Apple Support

If this drive is in a an incompatible format, stop running Photos with it immediately!  A Photos Library can sit on an incompatible drive, but running it may corrupt the database.

You might also want to check the permissions for your new drive, if you're sure it's properly formatted.  This is not usually a problem, but it sometimes comes up. 

Click on the drive icon, and open the Info window with command-i. Go to the bottom:

If “Ignore Ownership” is there, then it should be checked.

It seems like you can Read, but you can't Write-- is that correct? Can you edit already existing pictures?

tdresscher wrote: …The disc is formatted in EXFAT and connected to the mac with a cable. So that could be the problem.

Definitely!

I can indeed read but not write. But did not try to edit pics.

Don't try to edit until after you reformat!

the info window only shows 'you have custom access' at the bottom.

It will look different when formatted to APFS. For a mechanical drive, APFS or Mac OS Extended (Journaled) are OK. 

tdresscher wrote: … I realised that I always used to format discs specifically for Mac

Macs read ExFAT OK, and it works fine for static files like you might put on a Flash Drive. But it's not good enough to keep up with all the background work that goes on with a dynamic system like Photos Library.

I want to import a bunch of photos into it from an icloud photos library but I do not wish to associate this disc/library with icloud. I can't figure out a way to do this without losing date/time data. Any ideas?

I'm not quite sure what you want. If I wanted to download all the pictures from an iCloud Library, I'd make a new, empty Library, make it the System Library, sign in to that account, connect the new Library to iCloud, and let it fill.

Make a New System Library connected to iCloud - Apple Community

I'd not only get the pictures, but I'd get all the metadata, including edits, comments, keywords, etc-- the originals and all the data that Photos uses to make the edited versions. Then I'd sign out and reconnect to my main account and main Library. Then I'd deal with merging them.

If I just wanted a few pictures, I'd sign on to iCloud.com in that account, and then I'd select and download the ones I wanted. I'd have to choose if I wanted Original files or edited versions. I'd get copies with the correct dates, but not all of the metadata and other info that's in a Photos Library.

You mention dates--there are lots of dates. Finder, being a File Manager, shows when the file was created. Photos, an Image Manager, will show the date the Image was created. So will Preview and other Image editors.

Does that help?

tdresscher wrote: …if I download originals, I always get the creation date in finder, so I thought this was the problem there.

Right, because you're getting the original file so you get the original date. But the creation date stays with the image even if the file date changes. That's why we use Photos instead of Finder to organize pictures.