If you get an alert from Small Business Server stating :The following disk is low on free disk space. ……Drive letter: HarddiskVolume17″ (in my case) this is typically caused by a usb drive being unplugged without going through safe removal. The alert will get sent every day until the server is rebooted. You can also reduce the chance of this happening again by optimizing the drive for quick removal as per this post on Experts Exchange