I had this same problem - going through your great first solution didn't help at all. And Sansa didn't seem to have the correct firmware for the system.

However, in true RTM fashion, I installed Rhapsody, the suggested syncing software. I don't like Rhapsody and it doesn't seem to work all that well, but hey, it works to transfer.

For me this is a workable solution - we basically got this so my husband could have a cheap, larger capacity MP3 player for his audio books on business trips. We can still use WMP for our older m200 series Sansa players (which are also serviceable for our workout needs)

What i would recommend you to do is to uninstall the device and reinstall it back...to uninstall it...assuming that you are running windows xp...click on start, right click on my computer and go to properties..you should have your system properties open then click under hardware then device manager...look for the sansa device..it usually located under portable device..if its not showing there i would recommend you to look for under disk drives and see if its there..right click on the device and click on uninstall...after you unintall it unplug your device from the computer....Then follow this link below to download the porting kit...http://www.microsoft.com/downloads/details.aspx?FamilyId=A2E73160-E862-4F19-BB26-C0CAFE798955&displaylang=en .Then check the mode of yuor device to make sure it sunder MTP not msc...To check the mode go under menu, settings and choose usb mode...After you check the mode open up your windows media player click on tools, options and click under privacy...clear the history and caches...after you finish you can plug the device on the back of the computer not the front then windows should recongnize it and it should work properly..if that doesnt solve the problems upgrade your windows media player to 11 even though you have done that already..and try to reconnect your device..make sure you have it connected on the back USB>>>

And if that doesnt work try to obtain the driver for your sandisk device go to http://www.sandisk.com/DriverDownload/index.asp and find the driver for your device..and then connecct the device on the back of your computer...it should work properly..if you are still having any problems..let me know..i would be more than happy to assist you..
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Computerrep