Some useful hints
I used the last method (UMDGEN.COM - How To Make Your Own PSP UMD Rip .iSO Files - tutorial) with success. The only thing you have to worry about is to replace your deleted files with a dummy with the same name.
for example, in a certain game (1.3GB) there is a .pad file of more than 500 megs. It is very likely that such a file does not interfere with the game itself (i suspect it is an empty shell to counter illegal copying...). just delete this file, replace it with an empty one (same name), and you have a great chance of a working rip.