It’s best not to erase files, simply move them using the LR Library. If you erase files, all your editing (develop settings stored in the catalog) will be lost.
Simply connect your external drive and drag folders in the left hand panel from Mac HD to External HD. Or better still copy the folders and then update the location – see instructions in link below – figure 8. Once you are happy all links are updated, you can then delete everything from your Mac HD.
In future when importing, choose your external hard drive in the right hand panel of the import dialog. Or copy your raw files directly to a named folder on your external hard drive and then import into LR using:
Import >> Add
http://www.computer-darkroom.com/lr2_find_folder/find-folder.htm