I have been having a repeated problem after changing computers. I switched from a Windows laptop to a Mac. I have one LR catalog but different folders. The photos are saved on external hard drives. I use Onedrive for backup. After moving over to the Mac I had the exclamation marks showing that I had missing photos. I went through and matched all of the photos up. I only had about 400 missing which I knew where they were and left them missing because I didn't need them.
The problem occurs that everytime I close lightroom and open it again the photos show the exclamation marks and show the photos missing. I have the same external hard drives plugged into the same place. I have not renamed them at all and I have not added any photos to them. I have tried to find one just to make sure it works and it does work - it is there. I do not want to have have to go through the process every time I go into lightroom. I would like to find a solution to this issue. I have attached photos to better explain the problem.
Your Windows computer used drive letters to define hard drives, your MAC uses names. Your old catalog created on the Windows computer referenced the drive letters as part of locating the folders the photos are in. MAC fouls that use of drive letters up. Not a huge problem to fix. But relocating each photo is not the way to go, you need to update the folder locations.