cfitz, the Multi Mounter doesn't appear in WinXP, so your report is undoubtedly correct. I was using Win98SE when I copied the help file information. I find it odd that Nero wouldn't mention this operating system dependency in their help file.
So, depending on which operating system yongwc is using, he may or may not be able to use the Nero Multi Mounter.
yongwc, as cfitz suggests, if you are using Win2000 or WinXP, you may have to use IsoBuster to access the sessions. The simplest way to eliminate this problem in the future is to link
the sessions on the disc. When this is done, the multiple sessions will be read all at once without difficulty.
To link the sessions in Nero, when you choose "Continue Multisession disc", be sure that the "Replace files in compilation" and "Add new files to compilation" boxes are checked. This will ensure that multiple sessions are linked, and no special reader is necessary to access all files. I would also recommend that you read the Nero help file regarding multisession discs so that you understand all of that available information.
yongwc, after rereading your initial post, it occurs to me that you may have already been using the settings above (which should be the default settings). When "Replace files in compilation" is selected, it probably unlinks the file in the last session to the file with the same name in the earlier session. If you want to keep all copies of files in a linked session, you should probably rename the file before saving it.