This can happen with disks that are formatted as ntfs. This article is all you need to fix it. The external hard drive shows up as 'read only' on mac, help
External Hard Drive 'Read Only' on Mac? Fix It Now! (3 Ways)
hi there, do you know how to make the external hard drive accessible again when it shows as 'read only' on mac?
An external drive that is read only on mac is a very common issue with proven solutions
