You will have to repartition and reformat the External HD to move larger files to it.
For Intel iMac's set the External HD's Partition Map Scheme to GUID Partition Table and the Format to Mac OS X Extended.
See >
http://www.kenstone.net/fcphomepage/partitioningtiger.html
Warning: make sure you still have all you data on your Internal HD, because repartitioning and formatting will
Erase all the data on the External HD.
*Warning #2:* it also makes the drive unusable with a windows PC.