Time Machine, control retenetion

Hi,

I have two macs, one iMac and one MBA. The iMac has a 500 gb usb drive connected to it with time machine set up. The iMac is sharing this drive on the network, so that the MBA can take advantage of the same disk and also use that disk for Time Machine (TM) backups.

In TM I see that the total disk space of the iMac is around 120 gbs after exlusions, and the MBA has around 80 gb after exclusions. This would totalt to 200 gb on the usb disk, so I would have around 300 gbs left for incremental backups. The problem is, i constantly get a "TM is full" on both macs. This is due to TM not having the option to use "x number of copies" or "3 months back" feature. Its just dumping the files every day.

So, is there a way to control the number of copies or keep the retention a little lower? Any help greatly appreciated. Regards