This guide allows you to softmod your PS2 fat model in easy steps without an existing FMCB card or any rom manager. Thanks to the Reddit user u/quisys for the original guide. Disclaimer: make sure you know what disk you're writing to when using dd command, I am not responsible for any data loss.

Required Hardware

• Fat PS2
• Memory Card
• PS2 Network Adapter
• Controller
• HDD
• USB key
• SATA to USB cable

Required Software

• dd, exfat-utils, util-linux (you probably already have these in your distro)
• wLaunchELF KHN HDD image
• FreeMCBoot installer
• wLaunchELF 4.43x_isr
• OpenPS2 Loader

Install FreeMCBoot

• Connect your HDD to your computer, then write "__mbr.raw" (wLaunchELF KHN HDD image) to the HDD: sudo dd if=__mbr.raw of=/dev/sdX [Determine the path of your HDD using lsblk. It should be /dev/sdX, with X being the letter of your disk. ENSURE YOU USE THE PATH OF YOUR DISK! IT SHOULD NEVER BE /dev/sda]
• Unmount and remove the HDD
• Extract FreeMCBoot Installer then copy BOOT.ELF, OPNPS2LD.ELF and FMCBinst folder to your USB key
• Plug HDD and USB key into your PS2, then turn on the PS2
• Enter file browser, go to mass: and launch BOOT.ELF
• Enter file browser, go to mass: > FreeMCBinst folder and open FMCBInstaller.elf (not the exfat one), then select Install FreeMCBoot following the on-screen instructions. Restart your console
• Open wLaunchELF and go to file browser, go to mass: and hover over OPNPS2LD.ELF and press R1, select Copy. Go to mc0: or mc1: (depending on which slot your memory card is in), go to OPL and press R1, select Paste to overwrite OPNPS2LD.ELF
• Restart your PS2, open OpenPS2Loader, go to About and make sure the version is not 1.0.0, then turn off your PS2 and remove the HDD

HDD Management

• Connect your HDD to PC and format it in ExFAT (I suggest you a partition manager, or you can use mkfs.exfat /dev/sdX)
• Make a folder called DVD and a folder called CD in the root of your HDD
• Copy your videogames .iso to the DVD folder or your .cue/.bin to the CD folder

OPL Setup

• Plug in your HDD to the PS2, turn on your PS2
• Go to OpenPS2Loader > Settings and turn on cache game list (HDD), set BDM start mode to Auto
• Go to BDM Devices, disable USB, enable HDD
• Press OK, hit Save Changes