CD's not reading

Im using 10.5.4 on a PPC G5 Dual 2.5,
my optical drive is a SONY DVD RW DW-U21A (firmware AADB)

Interconnect: ATAPI
Burn Support: Yes (Apple Shipping Drive)
Cache: 2048 KB
Reads DVD: Yes
CD-Write: -R, -RW
DVD-Write: -R, -RW, +R, +RW
Write Strategies: CD-TAO, CD-SAO, CD-Raw, DVD-DAO

Some DVDs simply will not work, it sounds like the disc is spinning up, but eventually after 15 seconds, it just spits it out

Any suggestions why?

have you tried the normal stuff? zap pram, fix permissions, restart, etc?

you could also try another dvd brand.

If I remember back, some DVD drives would only use DVD-R if I am right, whereas the newer drives can read/write both the -R and +R versions.

All my old backups to DVD were on the DVD-R version and the newest backups are on the DVD+R. As suggested try another brand name and see if the -R version works.

If you are trying to read a +R version, perhaps updating the drive to a newer version would solve the current and future problems. DVD drives are very cheap now. I seem to remember buy only Pioneer drives for the older Macs but memory can fail me...