If you've defragmented your games properly, you should not get a yellow screen.
I have the problem with the open ps2, I've put the motorstorm arctic edge on the hdd [ connected via usb, formatted to fat32, defragmentated with game on it ], and it always froze on the yellow screen when I want to start the game, the same situation is with the god of war 1 and 2. Motorstrom is working like a charm on the usbadvance so maybe I've done something wrong with openps2? I've tried compatibility mode. I have to add that all my games are in PAL system, may it be a reason?
Ensure that your disk is corruption-free (use chkdsk/scandisk to check for errors).
Try re-installing you games again... if they still don't work. Repartitioning + formatting are the last options.
Also, when we say that you have to defragment your disk, we mean there should be no "Fragmented" regions displayed in your disk defragmenter (Some defragmenters just leave certain parts of the disk fragmented at times)!
OPL should NOT be region-locked, so you games (NTSC or PAL) should all work, provided that they were compatible in the 1st place.
USBAdvance/USBExtreme are able to play fragmented games, as they are not reliant on the disk's filesystem to be defragmented.
If all else fails, you may have the same issue as me: OPL does not like your USB disk, and will never work with it (All versions of it from 0.41 - v0.6 won't work).
Unmodified SCPH-77006 with SM 3.6
SCPH-39006 with M-chip modchip, SCPH-10281 NA and refurb Seagate 80GB HDD
SCPH-10000 v1.00 with SCPH-10190 PCMCIA NA and SCPH-20400 HDD unit