Apple Time Machine Not Filling Disk

[K9 Kai Niggemann]

Same situation here,

my disk has about 50 gigs of free space, still TimeMachine complains  
and says it's deleting old backups. But it warns me.

I have started to wonder: how do you restore the drive in case of a  
complete failure of the source drive? is there a "restore me" button  
somewhere?

Or is TimeMachine only meant to provide with an "undo" function after  
deleting individual files?

Kai