Comment 14 for bug 49890

The automount magic is not directly tied to power consumption, etc., but it is possible that the effect happens because at the time you connect the drive, the connection is too brittle for a robust file system detection and mount. Maybe it settles down a bit later when all the capacitors get fully loaded and power consumption reduces a little?

In either case, whenever an automount doesn't work, you should usually see some messages in the "dmesg" output (also reflected in /var/log/kern.log), most often USB port resets.