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."