The method you used is the only reliable method on a FAT PS2.
Pulling out the drive-bay whilst the disc is spinning is not a good idea (because of the drive-mechanics) and since these game-swap-methods are time-dependent, it is good to have the drive/case opened...
I guess you did something wrong with the image/game/disc-creation/preparation.
PS2 V7/DMS3 V2 (FW:2.4b7); Seagate Baracuda 200GB
PS2 V7/CC1.0 (FW:34 hacked v2 BM:2.1.6); Maxtor DiamondMAX9 PLUS 160GB
PS2 SCPH-30004R; NoMod+NoLaser
3xSony MC 8MB
MAX/Datel 16MB with Boot-CD
Custom FMCB 1.8b+ Beta-Build, my AIO 0.5, Sony&xRhino-Linux