I'm having a strange (at least for me) problem burning multisession DVD+R media: the dvd becomes unreadable after the 3rd session is burned.
I have all reasons to believe that it rather has everything to do with position of last session, than with the exact number of sessions. I also have all reasons to believe that it's rather ide-cd.c bug than isofs. In other words this problem was already reported to me, but I didn't have time to bring it up with linux-kernel people yet.
mount refuses to do its work, and kernel says: Unable to identify CD-ROM format. Note that there isn't any read error, so the kernel is simply unable to locate the primary volume descriptor.
The keywords for this problem are:
^^^ ide-cd.c is involved [it's no problem with sr.c if unit is routed through ide-scsi.c]...growisofs -M /dev/hdc -J -r <files> (-Z for the first session)
^^^^^^^ ... and last recorded session starts beyond LBA #1152000, which corresponds ~2.2GB.This is the output of dvd+rw-mediainfo: ... Multi-session Info: #3@1339392
What's so special about 1152000 (besides that it reminds highest posible bitrate for serial port:-) It's 256 times 60 times 75. What's so special about these numbers? 256 is amount of interger values which can be represented with 8-bit number, 60 is amount of seconds in minute and 75 is amount of frames in one second of CD-DA. Yes, it's about conversion from MSF to LBA suffering from overflow around 2.2GB. In the nutshell the problem is that drivers/ide/ide-cd.c always pull TOC in MSF format and then attempts to convert it to LBA. If last session is recorded beyond 1152000, isofs driver will be led by ide-cd driver to belief that volume descriptor resides at 1152000, which in turn results in "unable to identify CD-ROM format" message logged upon mount attempt.
As fast-acting remedy I can suggest to route your unit through ide-scsi. The way it was under 2.4. Even though it's declared unsupported it actually still works in 2.6 (I for one still use it). And once ide-cd.c is fixed you'll be able to revert back to officially recommended path. A.