My friend Allison Sheridan sent out a tweet for help that struck a chord with me. While on vacation she had spotty Wi-Fi coverage and was using her Mac tethered to a Mi-Fi hot spot with a limited data plan. Like me, Allison uses CrashPlan for automatic offsite backups, but apparently her CrashPlan backup was chewing up her data allowance. Allison thought she was connected to hotel Wi-Fi but because her coverage was spotty the Mac reconnected to her Mi-Fi and CrashPlan was using her precious bandwidth and churning away. It wasn't until Allison got notification of the data use she realized something was wrong.
Fortunately Allister Jenks and SwitcherMark shared a tip on Twitter to avoid this type of thing from happening in the future. Turns out, you can tell CrashPlan which networks not to use for backup, but you have to configure it in advance. This can be found in CrashPlan Settings > Network and selecting the button to configure wireless networks. Here you can deselect wireless networks you do not want CrashPlan to use for backup, notably any tethered devices or hot spots. The list may be extensive so make sure you scroll through it.
I understand why CrashPlan made this an "opt out" rather than "opt in" setting. They want your backups to run as often as possible, especially when you're traveling and may be more pone to lose you computer. I personally prefer CrashPlan would look at an "opt in" model to allow you to select which networks you want to use, it's still nice to have the option to configure this.