Bypassing the Failed Disk "Read Only" Mode
Have you tried repairing the disk from recovery HD or while booting into MacOS from a different hard drive? 

If you truly want to be able to write to that disk again I would recommend this attempt: Clone the HD to a different HD. Check the health of the new HD and make sure all the files are accountable. Then reformat the previous HD, check the health then put the files back...   However, the disk may be writable again, it’s not going to be reliable. 

If I were you, I would get ready to retire drive.