Can you please share what have you tried already as troubleshooting methods to remove the write protection?
I'd recommend you check if you will be able to fix the permissions for the external HDD through Disk Utility. Here's a tutorial on how to test your external drive for defects there.
If it doesn't help, I'd advise you check the suggested answers in this thread from the community .
Hi, many thanks for your very prompt reply - it is much appreciated. I did the First Aid changes you recommended, but they did not solve the problem. However, the first/second reply in the user community you refer to below DID work on my 2012 MacBook Pro. So thank you so much!! For others with the same issue I had, please can you give them this text answer below? It will save them much anguish. Thanks.
"Right click/ctrl+click on the Hard Disk (HD) icon and click "Get Info". You can also select the HD icon and press command+i. At the very bottom of the window that comes up there are read/write permissions. Click the lock in the bottom right hand corner to make changes to read/write permissions that fit your needs.
(Notes: You need the Admin password to make the changes above. This works for me on OSX v10.9.2; not 100% sure if it works on older OSX versions.)
If you can't find the HD icon, enter the name of the HD in Spotlight search. By default mine is called Untitled."