From Charlie's FAQ and Guides page
PS2 backups/imports (CD-R/DVD-R) won't boot or Red Screen Displays
Disk was seen (groove found & focused) but PS2 was unable to identify it as valid disk type (Audio/DVD/PS1/PS2). Note that if the groove had not been found, the Browser would have displayed No Data.
If itÂˇÂ¦s all backups that wonÂˇÂ¦t boot, first suspect is B C, then try cleaning the laser lens with a cleaning CD. If itÂˇÂ¦s just DVD-Rs that wonÂˇÂ¦t boot, suspect wires E, R, W or the DVD laser diode (which may need recalibration). Laser Azimuth might also need adjusting.
Then suspect a loose wire, solder bridges or solder balls. If you canÂˇÂ¦t see bridges/balls, then itÂˇÂ¦s the hard way. If the BIOS wires are attached, detach them from the BIOS & clean up. Boot a backup using AR2/GS2. If it works, A-I are OK; the BIOS was the problem. (You can try this with the BIOS wires still attached but if it doesnÂˇÂ¦t work, youÂˇÂ¦ll have to take the BIOS wires off).
When you can boot backups (including DVD-R) with AR2/GS2, carefully re-attach the BIOS wires, MNOP QTUV RW. Note that R and/or W may be located off the BIOS chip on certain PS2 models. Then try direct booting of backups and/or imports.