Cannot automatically backup on schedule because of huge temp file

I have Acronis True Image Home 2011. Let me say first that, although this is a good product, I do not recommend it to my friends because you don't offer appropriate customer support. Pay per incident is lame. Sorry. You think you are saving money but it costs you in customers.

Now to the problem. I back up my large C drive of 500 GB and a D drive of almost 1 TB. I have external drives which are large enough for the backup files. However, the program won't overwrite the old backup files. First it must create a huge temp file which exceeds the size of the external drive which is almost full due to the previous backup. If it just overwrote like it did in the past, I'd be fine. Instead, I need an external drive almost DOUBLE the size of my backup file. So I have to go in manually and delete the old file. This means I can't schedule it etc. I need an option to start overwriting without the temp file.