Problems with linking iPhoto to external drive files

In order to back up my iphoto albums, I mistakenly put the original files on an external drive rather than dragging the iphoto icon to the external drive. Now I cannot directly open those files located on the external drive from iPhoto on my laptop. I tried linking to the drive (pressing alt/opt when opening iPhoto and selecting the external drive), but iPhoto on my laptop won't read it. I don't have enough space now on my internal drive to drag all the originals from the external drive back to iPhoto and then drag them properly to the external drive. I could do the file "reference" option, but everyone seems to recommend against that since it potentially complicates future actions. What do you recommend for making iPhoto on my laptop properly read the original files now placed on an external drive?

Thanks in advance,

Richard