(try downgrading webman to 4.85.01.........try using/downgrading to prepNTSF_1.17 (for external hdd with ntsc format with .iso games)
Webman has a setting in the options to use less memory, that, is what worked for me, in booting some games that wouldn't off .iso on external hdd.
(webman has a known issue of freezing if you keep loading and unloading .iso games, they said it was fixed on a newer version, but.............i had to load a bunch of games in order to get it to freeze that was on a previous version)