If you are using a SATA/IDE Hard Drive with OPL (Open PS2 Loader), you may need a specific "patched" version of Codebreaker 101 that recognizes the hdd0:/ partition.

A verified Codebreaker 101 ELF should typically be around 3MB to 4MB in size. If the download is an .exe file or significantly larger, delete it immediately. How to Install Codebreaker 101 on PS2

Version 10.1 (often referred to as Codebreaker 101) is widely considered the "Gold Standard." It is the most stable version and supports a massive library of games. Where to Find a Verified Download