took me ages to get this after MUCH searching, i still had to work it out myself... move PSN games from internal HDD0 drive 'Game' folder, e.g. 'hdd0:/game' to an external drive in a folder called 'GAMEI', e.g. 'USB0:/GAMEI'
use multiman for this, and only move PSN game. I haven't the drive in front of me, but am sure they start with NP or something
anyway avoid moving BLES BLUS folders from hdd0/game, they are savegame data and trophys i think.
when they were moved, the Game in XMB are all grey controller icons. i found i already have an OLD version of gamedata (it has a USB icon beside it) but i downloaded the latest form the download column (far right) in multiman, and installed that package in the usual way.
so after it is installed, i ran gamedata and after a 'reboot' all the grey icons turned to the proper game icons in XMB and i can play what i any PSN from the external drive (the game folder earlier)
i've seen MANY guides all about making extra folders, or running this gamedata usb icon BEFORE installing some games, but if you already have them installed already, this is best.
for new PSN games, reboot to get the 'grey icons' back, install the game, (it'll be to only colour/animated one), then go to multiman, move it to external, it'll go grey like the others until gamedata is run to get them all back (ie it re-directs the icons lookup to the external)
hope that helps, the phrases might be a bit... 'off' so PM if you need more help