Compressing a PSP ISO into a PSP CSO: (Don't let the number of steps scare you, I simply broke it down into greater detail). 1.) Download UMDGen v4.00 from: http://umdgen.ps3news.com/UMDGen_v4.00.rar 2.) Open UMDGen. 3.) Click the "Open" button from the top menu buttons. 4.) Locate the PSP ISO you want to compress and click "Open". 5.) The info of your PSP ISO should be displayed. On the left-most box, click the "+" icon to collapse the "PSP_GAME" folder into two more folders: "SYSDIR" and "USRDIR". 6.) Click on the "SYSDIR" folder in the left-most box. 3 objects should now be displayed on the right-side box (these are the objects inside of the "SYSDIR" folder). 7.) The three objects should be: A folder named "Update", a file named "EBOOT.BIN", and a file named "BOOT.BIN". We're only concerned with the "Update" folder and "BOOT.BIN" file. DO NOT mess around with the "EBOOT.BIN" file, it is needed to run the game properly. 8.) Right-click the "Update" folder and select "Delete". Do the same to the "BOOT.BIN" file. This will delete the Update folder and BOOT.BIN file, which both take up a significant amount of space, but are not needed at all when running an ISO or CSO. Deleting this folder and file should shave a good deal of MB's off of the final size. 9.) Once the "Update" folder has been deleted, it's time to compress your ISO into a CSO. Click the "Save" button from the top menu buttons. 10.) You should be presented with 3 options: Uncompressed (*.iso), Compressed (*.cso), and Compressed (*.dax). Go ahead and click on Compressed (*.cso) to setup the compressing process. 11.) You will be presented with a "Save As" screen. Choose a location to save your new CSO, enter a name for your CSO (can be anything, doesn't have to be the same name as the ISO), and click "Save". 12.) A new box should now pop up with a drop-down menu asking you to "Please Select a Compression Value:". Set the Compression Value to "9" if it isn't already (ALWAYS, ALWAYS make sure it's set to level 9 so that the maximum amount of space is shaved off during the conversion process). Click "Continue". 13.) A progress bar will pop up. This process will take a while depending on how big your original ISO was. Once it reaches 100%, it will start another progress bar from 0%. Don't freak out, this is normal. The first progress bar tracks the copying of the original ISO files, the second progress bar tracks the compression process. 14.) Once both progress bars are complete and gone, you're done! Close UMDGen, go to the location you chose to save the CSO to earlier, and you should see your shiny new CSO! Note that your custom name is the filename and that the CSO size is significantly smaller than it's previous ISO size. 15.) Simply take your new CSO from it's saved location and copy it over to the "ISO" folder on your PSP. You're good to go!