When you add a new monitor to the Device Templates, such as Disk Usage, it will remove any manually added performance monitors and remove all the thresholds.
The test. Added a monitor to a Linux(Any device works) Server (Delivered Mails) Went to "Device Templates" --> "Linux" and add "Disk Writes" "Apply" to all, the default. Go look at my Linux server. The "Delivered Mails" monitor is now gone. To make this work, all the Thresholds are gone!