If the files were ripped in iTunes they are probably .m4a or .aac–Apple formats that the Sansa won’t play. iTunes hates you for not buying a doPi player.
Take those Page/Plant/Jones/Bonham classics off the player. They’re not gonna play. You need mp3 versions–and you can get iTunes to make them.
I’d suggest you start from scratch and re-rip the Zep albums to mp3.
You can set iTunes to just rip mp3 files from the get-go, which will make your music a lot more portable because everything plays mp3. Under Edit/Preferences/Advanced in iTunes (not the more visible Advanced tab) you change the Rip setting to make mp3 files. Give them high quality–at least 192 kbps and hey, don’t you think “Kashmir” should be full 320 kbps? No more incompatibility problems.
Or if for some reason you don’t want to re-rip the CDs, go into your music library in iTunes, highlight the Zep files, right-click on them and Create mp3 Version. Now the trick is to find them on your computer–figure out where iTunes has created your library. Put the mp3 versions in separate folders and transfer those to the Sansa.
Another thing you can do is look at the player once the mp3 albums are on there. Search the player for MACOSX. Those are the folders of Milkerman’s “ghost” files–finder files–that Mac needs but Sansa doesn’t. If you look inside them you will see that they are tiny 1kb files with nearly the same names as the songs plus underscores. Because they are labeled mp3, the Sansa may try to play them, but there’s no music in them to play.
You can (and should) delete all the MACOSX folders from the player.