Override 15% free disk space limitation in RnR

[Fizz]

I am trying to backup my main C: to an external USB drive. I use RnR 4 on a T60. My C: has 50 GB of data and I have more than 60 GB GB free on my external drive. However, RnR fails with an error stating that RnR needs at least 15% free space on the backup drive.

Is there anyway to override the 15% free space limitation?

Thank you.