ziso and maxcso are programs. You need ZSO header and image compressed by LZ4. CISO is not supported and this is default compression used in maxcso.
https://github.com/unknownbrackets/maxcsoLZ4 support is mostly for experimentation.
actually i use this version OPNPS2LD-v1.2.0-Beta-1968-1b8efe7 think is the new bdm4, making more test with a usb drive and zso load faster, but plug in a mx4sio performance is worst. im using a 90001 slim ps2 with Funtuna.Which OPL build? BDM4?
Basically any data within the ISO that's already compressed or doesn't make sense to be compressed (compressing it doesn't gain anything). So yeah you're gonna see chunks of raw data. Most compressed formats support these so called NC Areas (Non-Compressed Areas), such as the CSO/ZSO standard, DAX (version 1) and JSO.@Acid_Snake
If I may ask, what data is being compressed with the ZSO format? I looked through some compressed games in HxD and was able to find uncompressed data such as the ELF file (7F454C46).