move your PlayOnMac folder to your external drive and make a symbolic link
your PlayOnMac folder is in ~/Library
example your external drive called EXTERNAL
open terminal
move the folder to external drive with
mv ~/Library/PlayOnMac /Volumes/EXTERNAL/PlayOnMac
make a symbolic link with
ln -s /Volumes/EXTERNAL/PlayOnMac ~/Library/PlayOnMac
cheers
Edited by webflat