I have a 1.5TB external HARD DRIVE that I am using for time machine to back up my 500GB internal HD of my IMAC operating snow leopard.
When up I try to start Time Machine it tells me it failed because the amount of GB needing to be backed up is 582GB (don't remember exact number, but it is more than the internal HD. No other external HD are available. The last successful backup was 8 months ago. The 1.5 TB HD is almost exclusively devoted to Time Machine.
I don't understand why it wants to backup MORE than the size of my Internal Hard Drive, especially since it has previously deleted old backups, and I have been using Time Machine since 12/2012 when I bought my computer. It is not like I have made major changes requiring a 100% backup of the IMACs Internal HD.