My time machine is fairly new bought in Sep12. The time machine is suddenly say that it can no longer back up as the data is 'read only'?

Hello

Today I called Apple is I needed to access time machine. The Apple rep assisted me and I thought all had been sorted. However, just a few moments I tried to ensure that Time Machine was still being backed up - time machine's pop up note said 'files can't be copied onto the back up disc because it appears to be read only.

I am using a MacBook Pro bought in Sep12.