I have encountered the same behavior as is described in this thread:
In my case, this problem happened after recycling the power only the router connecting my 2 My Book Live devices. There were no power interruptions for either the source nor target MBL devices.
Rebooting the SOURCE My Book Live did resolve the issue (the Safepoint did reappear within the list).
I am running the latest firmware (MyBookLive 02.32.05-046 : Core F/W) on both devices. I feel the firmware should be more tollerant/resilient of network interuptions. This seems like a bug to me. If the source is for some reason unable to reach the destination, the Safepoint should continue to be listed in the UI but should display an error condition such as “TARGET not found” or something like that. Also, I think that the Source should be able to retry connecting to the target automatically without the need for a manual reboot.
I feel as though this disappearing from the list behavior might be something new in this latest firmware, because I only started noticing this problem since last Sept 2012 when the current firmware was pushed out.