I've got a portable hard drive connect to my iMac and have been running Time Machine successfully for a long time. However over the last month I've been getting the error message that it won't back up because there's not enough space. I thought the idea was that it would delete the oldest ones when there is no longer enough space, but it appears to not be doing this. Is there a setting I need to change? Please help as I've had no back up since 31 December according to the error message!