Can't create a new folder on my Seagate Backup Plus Drive after updating operating system to macOS Big Sur

I recently had to have my MacBook Air hard drive wiped at the Apple store. I backed up everything on my 2 TB Seagate Backup external drive. I'm a photographer so I transfer large files and have been using this drive for years with no problem. Now I can't create a new folder on my Seagate drive, so essentially I cannot transfer any new files to it (or further organize what's already on it). Help! I'm currently running on macOS Big Sur. Everything else is working great and I can still access my images via Lightroom. I just can't organize anything anymore which is an essential part of my workflow. I also can't reformat my Seagate drive because it IS my backup and the files are too large to transfer back to my Mac. What do I do? Is buying another external hard drive my only option to move forward??? I have holiday images I need to edit and deliver to a client ASAP!