I have an external drive that I use to back up photos at the end of the year. When I plug it into my Mac, I can see it and do anything except write to it. If I right-click on it, Get Info shows “you can only read” under Sharing & Permissions. What do I need to do to write to it?
What file system does your external drive have? If I’m not mistaken, I believe your drive is formatted in NTFS.
What you described perfectly fits how an NTFS drive will perform on a Mac computer.
To check what file system your drive has, open Disk Utility and select your drive from the left side. Then you should see the info at the right side.
If it is indeed NTFS, use iBoysoft NTFS for Mac to enable write support to NTFS on Mac.
There are various situations that may render you unable to write to an external hard drive on a Mac. Read this comprehensive guide if you want to learn more details: Can't Save/Write to External Hard Drive on Mac (7 Solutions)