I've had the same problem with external USB & Firewire drives; you eject them using 'disk utility' (perhaps by mistake) only to have to physically disconnect/reconnect the drive in order to get Mac OS X to see them again; despite them being visible under 'System Profiler'.
This is more than an inconvenience if using a daisy-chain of Firewire HDDs...
Here's what worked for me; try this from within Terminal.
(1st make sure you have 'diskutil' installed - see note below.)
diskutil mountDisk /dev/diskx
where diskx correlates to the physical device reference as displayed under the "list" command e.g. disk2
NB: try MacPorts for http://www.macports.org/