A common problem when plugging in USB and FW 800 external hard disks under OSX 10.5 and 10.6 is that the volume mounts (a disk appears in the Finder sidebar) but the disk cannot be opened. The following message appears:
“The alias can’t be opened because the original item can’t be found”
Neither deleting or fixing the alias works. The solution is very simple if you have XCode installed.
Open the Terminal and type (or copy from here):
/usr/bin/SetFile -a a /Volumes/<YourDiskName>
You can see the name of the disk (substitute for <YourDiskName>) in the sidebar of the Finder.
This problem seems to have disappeared in Lion (10.7). Thank goodness.