Time Machine-not enough space
I started getting messages that there was not enough space on the external hard disk I use for Time Machine. I tried to trash older files but I was having difficulty, so I used Disk Utility and erased the disk. I pressed back up and I now get a message "Preparing". I guess this will take awhile, but will be functional again? I've used Time Machine for three years and never got the message "not enough space" before. Isn't it supposed to delete oldest backup to give space for the newest one? Is there a preference I need to check?
Thanks,
Stu
iMac 20 in., Mac OS X (10.5.8)