A couple quick things you can check. Make sure that you don't boot with the "noacpi" boot option. That can prevent 3 daemons from running that automounting depend on, hald, udevd & dbusd.
Before inserting an external device, open up a console and run "sudo tail -f /var/log/messages". Is the device and the partition both detected? The device will be assigned a node that looks like /dev/sdb or /dev/sdc. The first partition will be assigned a device node like /dev/sdb1 or /dev/sdc1. Sometimes there is a reason the device wasn't mounted automatically and this reason is printed out in the messages log.
If you logged out improperly, after a crash, or by using [ctrl]-[alt]-[backspace] instead of logging out, there could be a stale /media/.hal-mtab or /media/.hal-mtab-lock file. (Ubuntu may do this differently). If nothing is mounted in /media, you should be able to delete these files.
If the usb drive was unplugged before the cached writes were unloaded to the device, the filesystem may be corrupt. In that case, either the kernel won't recognise the partition, or automounting failed. This is similar to the bad-device possibility of the above post, but the filesystem is bad rather than the device itself.
I'm not certain, but I think on Ubuntu, you need to be a member of the hotplug group for automounting to be attempted. Could membership have changed.
Sometimes not all usb ports are created equally. You could try a different port. The most common problem is that a certain port doesn't supply enough power. A highspeed device may not work well on a port because the hub is being shared with a usb 1.0 device.
On my system, an entry in /etc/fstab for node will prevent automounting, because it seems that you want the device mounted somewhere else. Ubuntu on the other hand may be configured to modify /etc/fstab. If the device isn't mounted but an entry in /etc/fstab exists from when it was plugged in last, this may cause problems. This may have the same effect as a stale /media/.hal-mtab or /media/.hal-mtab-lock file. ( The entry in /etc/fstab serves the same purpose)
Last edited by jschiwal; 12-18-2008 at 03:00 PM.