This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
A common mistake when burning a multisession CD-ROM is to forget to link
the files from the previous session(s) into the current one. This results
in a CD-ROM where you can see the new files but none of the old, unless
you have a program that lets you choose which session you look at.
Most recording applications these days will ask you if you want to preserve
the data from the earlier sessions, or will default to keeping it. Earlier
versions of the software either defaulted to throwing it away or didn't
support the feature at all.
The files themselves aren't really lost. Some programs are available that
will let you access the "lost" data, including IsoBuster (section (6-2-20))
and CD-R Diagnostic (6-2-6). Some CD recording software will allow you to
extract the data track from a specific session, which you can then access
with IsoBuster or WinImage (6-2-2).
A more transparent solution is to use a "session selector", such as the one
that ships with Roxio software. This lets you choose which session you see
in Windows explorer.
Some older CD-ROM drives had a "feature" that caused them to stop looking
for sessions after a certain point, so if you wrote too many of them you'd
"lose" the data from the last session rather than the next-to-last. This
is also curable with the above solutions, though you may have to use a more