Locked iTunes library on external hard drive

I moved my iTunes library to an external hard drive yesterday to make more room on my Mac. Had it all set up and working well, I was able to access my new library fine yesterday. Today I've switched on again and it's not working.

When I click on iTunes the following box pops up:

"The folder iTunes is on a locked disk or you do not have write permissions for this folder"

I click OK and it asks me to choose a library.

I select iTunes Library.itl on my external hard drive and then the message is:

"The iTunes Library.itl file is locked, on a locked disk or you do not have write permission for this file".

When I 'Get Info' on my external hard drive, under sharing and permissions it is already for read and write for my Mac username.

Can anyone offer some advice please?!