Osamu Aoki wrote: > My post on this thread needs few errata: > 1. CD image size can be obtained by "mount" the CD and run "df". Interesting. Thanks for educating us about that. > 2. Only "readcd" run with nexact CD size can extract the correct image > 3. "dd" tends to copy most data correct but losews last few KB of data. I have been reading this thread with great interest because I had determined the following behavior. cdrecord image-in.iso dd if=/dev/cdrw of=image-out-cdrw.iso # works cmp image-in.iso image-out-cdrw.iso # passes dd if=/dev/cdrom of=image-out-cdrom.iso # reports errors on last few kb cmp image-in.iso image-outcdrom.iso # fails The images compare exactly when being read from the cdrw drive. But if I move the CD out of the writer that was used to write the image and put it in a different CD reader only, not a writer, then the repeat the dd then the image I retrieve is different and fails compare. The last few kb of data is missing. The dd reports an I/O error when trying to read those bytes on a cd reader only device. It was fine on a cd writer device. Having a number of machines and systems I repeated this test with both cdrecord and with MS-Windows Roxio software on the both the same hardware and on different hardware and got the same result. Both cdrecord and roxio had identical behavior. A cd writer drive could read those last few kb of data while a cd reader only drive could not. The cd reader only drive would generate I/O errors while trying to read the last few sectors. This seems to be either a common problem or a hardware behavior. I know too little to do more than report the behavior I am seeing. The behavior was "as if" the power to the writer for the last part of the data was reduced, thereby rendering the disk readable for those last sectors only by a writer drive. It seems to be independent of the size of the image written. So to test this you might as well use a small image created with mkisofs so the testing runs quickly. And use a CDRW so that there is no wasted media. I also tested with both CDR and CDRW media. There was no difference in behavior based upon different media that I tried. I tried both good quality and cheap quality CDR media. Same result. Different CDRW drives can all read the data completely with no errors. Different CDROM drives all failed reading the last few kb. I initially ran into this when I tried to verify that a used Phillips DVD-CDRW combo was working correctly. It drove me crazy for a while trying to isolate if the drive was really good or not. But the behavior is matching a new Plextor CDRW exactly. And different independent software is generating the same result. I imagine something fundamental is happening that I am not understanding. Bob
Attachment:
pgpz5sYyThCSR.pgp
Description: PGP signature