You have to use USBAdvance tools to install games to a USB HDD. But I will save you a ton of trouble and tell you it is a total waste of time. PS2 only has USB 1.1 and the data transfer rate equals in stuttering of 99.9% of all audio and video in most games. The network adapter and HDD is a much better way to go if you ask me. But if determined to see how bad it can be find the USBadvance tools to install the PS2 games. Copying over the image will not work and even then not all USB devices are supported. I use USB drives for small apps and such but out of my three only one works.
PS3 CECH-2501A [CFW] & CECH-3001 [OFW]/ PS2 SCPH-3901 & SCPH-7501/ PS1 SCPH-5501
Mod Cobra 4.46 v7.00 w/ E3 Flasher [CFW] & Cobra ODE 4.50 [OFW]/ Mod HDD-OSD [Phat] & Modbo 4.0 [Slim]/ Mod MM3
HDD 1tb [CFW] & 640gb [OFW]/ HDD 640gb & OPL v0.92 w/ 1.5tb SMB