Using my Dell desktop computer (Optiplex GX280), I successfully loaded music files onto my Sansa c140. However, when I connect the c140 to my Dell laptop (Latitude D400), I can’t see any of the music files, and therefore can’t add or delete any. Both computers are running XP Pro SP2, and have Media Player 10. I am using Windows Explorer to look at (or try to look at) music files. There must be some different setting between the two computers. Any clue what that difference could be?
Might be your laptop isn’t set to “Show hidden files and folders” in Windows Explorer.
Tools > Folder Options > View tab. 
Sorry I should have mentioned that, too. On both computers, I have selected to show hidden files, so that’s not the issue.
Well OK, how 'bout this?
Did you transfer the music via the ‘sync’ method in MTP mode and Windows Media Player. If this is the case, when you plug your player into a different computer, WMP won’t see the music that you synced on this hard drive and will erase everything on your player.
OR
Files transferred in one mode are not visible to the computer in the other. In other words, if you transferred music in MTP mode, you won’t be able to see in in MSC mode, and vice-versa.
Hopefully, it’s the 2nd possibility. If it’s the 1st, it’s gonna be a PITA re-loading everything again. 
All of the files were dragged and dropped in Windows Explorer. WMP wasn’t used. When moving the player from one computer to the other, there was no change made in the mode setting. But I should experiment with the different mode settings and will report back. Thanks.
I’m having the same problem in Windows Media Player with my c150 (same version, larger space). I cannot delete any files because WMP indicates “there are no files on this device” (yet it is over 3/4 full)
I’m having the same exact issue and was a little bummed out to see that no answer has been given.
Maybe if you try the MP3 Legacy boardwhere the ancient, retired players forums are hidden, you might find answers to your questions. At least there, people who have (or are familiar with) these series of devices hang out and may be able to help you.
