Here are 3 quick ways to do it and some bonus tips. Go to finder > applications > utilities > disk utility to launch disk utility. An external drive that is read only on mac is a very common issue with proven solutions
External Hard Disk Is Read Only In Mac at Harry Brawner blog
This article is all you need to fix it.
Is your external hard drive showing up as read only on your mac
Learn how to fix a locked mac drive and get it working again. Seeing the message you can only read beneath sharing & permissions If it's an ntfs external drive, use iboysoft ntfs for mac to gain instant write access to it on mac.
