Permission to write but NOT overwrite files on external drive

Hi,

since very recently whenever I try to overwrite an image (photo) file on an external hard drive, i get an error stating I cannot save the file because there is no access permission granted. It's happening primarily in photoshop. If I want to save the image with a new name, there's no problem. The problem occurs when I want to save a new version by overwriting the existing file. Now I need to save the file under a unique name, delete the older version, and rename the new version to the original file name.

It also happens when I download a new file which is sent to me through wetransfer. Again I get an error stating there is no permission to write to the external hard drive. The workaround for that is to save to the local hard drive and then simply move the file to the external hard drive.

Again, this is something which started happening recently. It could be as a result of installing Monterrey. Also Photoshop is a new installation.

Any thoughts on this?