How do I mount ejected external USB flash storage?
The problem you describe was initially the intention for me to write an app that allows you to manage your drives from the menu bar. It is called Mountain and you can get it here.

Hope you enjoy it!!

UPDATE: The only way to re-mount ejected external USB flash drives is to unload and load the USB mass storage kernel extension:

kextunload /System/Library/Extensions/IOUSBMassStorageClass.kext


Followed by:

kextload /System/Library/Extensions/IOUSBMassStorageClass.kext


Be aware that this will re-mount ALL external USB drives..