If you want to permanently disable the backup alarm, you can create a debugchumby script that contains:
rm -f /psp/ifalarm
If you prefer not to have a USB dongle attached to the device to disable the back-up alarm, you can insert the rm -f /psp/ifalarm command before the /bin/chumbalarmd statement in the init script by using the running something at boot without debugchumby method. However, please only use this method if you know -exactly- what you're doing, as you can cause a permanent boot failure which cannot be resolved without restoring factory settings or applying a USB firmware update.
A couple other things that may be of interest regarding the back-up alarm:
1. You can override the back-up alarm audio duration (though the screen will stay up until dismissed) by setting /psp/backup_alarm_duration in seconds. Default is 1800 seconds (30 minutes).
2. You can override the back-up volume by setting /psp/backup_alarm_volume to a value from 0-100. Volume defaults to 100.