Caviar Black 1tb suddenly read-only?

I have been using my Caviar Black 1TB for a while now and loved it’s performance, and of course it’s giant capacity.  However yesterday I noticed it is suddenly set to read only, and I cannot delete, move or rename files.  I can copy files from the drive to another drive, but that’s about it.  

Is there any way to put it back to Read and Write? As far as I know, i didn’t do anything to set it this way.  The only new thing is it is a bit low on space now (3gb).  Could that have done something?

Really concerned about this.  If I have to reformat, that’s A LOT of information I’m going to loose.

Back up your data ASAP (you should always have (including existing backups) at least 2 times more blank space to use for backups as you do data. Your likely going to need to reformat the drive, and if you do you might as well do a full Write-Zeros.