There seems to be some confusion when it comes to auto mounting in ArchBang. Users expect that on opening pcmanfm file manager any devices or drives will be automatically mounted. This is simply not the case for this you would need udisks2. Adding the package would more than likely push ArchBang over a gig something I trying to avoid.
Personally I just run sudo fdisk -l to view devices/drives and mount them as I wish.
Udevil package is on iso and it includes an application called devmon. You can easily test it by simply opening a terminal and running:
devmon
If any devices or drives are detected they will be automatically mounted to /media [as seen from devmon output]
Then you could just open pcmanfm and go to /media folder.
More information is available here:
Would not be too difficult to add as a autostart option in i3.
Update Am testing iso auto starting devmon and seems to work out of the box. External devices are mounted to /media