time machine doesn't overwrite backup...

hi mac users, i'm a novice mac user and need help with the time machine.
the problem is it doesn't overwrite old backup when i turn it on.


i have a 1TB external hd which i use to backup my mac. the mac backup is roughly 800gb, so i switch off the time machine as i don't want it on everyday of every month.... i want to turn in on/off every so often so the backup is up to date rather than updated daily if it was on all the time. however when i turn the time machine on again- it doesn't replace the old backup but uses the space left which isn't much.
how can i make it replace the backup whenever i turn it back on? i dont really want to go through disk utility to erase 800gb of stuff every so often....

Welcome to the Mac Forums.

The reason is that after the first full backup, Time Machine does an incremental backup. In other words, only the changes.

What you need to do each time is erase the previous backup and start over. Unless of course you purchase and use a larger external hard drive.

hi chscag, you say it only backups the changes but when i turn time machine on the file size is similiar to the original (800gb). i don't make 800gb aditional content as i specify what folders are copied. is there a better way than to wipe clean and rebackup as that is time consuming.

No, unfortunately that's the way TM works. It will always keep a full backup for you in place as long as there is room on the drive initially. That's why you're seeing the 800 GB. If I were you, I would either get a much larger hard drive or use a different method of backing up all that data. 800 GB is going to take awhile to backup no matter which method you use.

thanks chscag! very helpful advice