Accessing folders with a red minus sign

I have Sonoma 14.3.1 on an M1 iMac. I am the only user of my computer and therefore also the administrator.

I have copied a set of files to a folder an an external network drive (WD Ultra).

I can see that drive and the folder using Finder on my iMac.

But the folder is marked with the minus sign in a red circle on the bottom right.

If I look at File Info in Finder it shows, correctly, that I have Read/Write access to that folder (except that I don't).

I can unlock Sharing and Permissions (but I shouldn't need to) and although this lets me change permissions they always immediatley revert to their original settings.

How do I get rid of the minus sign and gain access to the folder on the network drive? (I have no problem accessing other folders on that drive which I created by copying files to it from other computers)

Mike