Random “no disk” and DRM errors are a sign that there may be a glitch in the device’s file system.
If you connect the device in MSC mode, where you have direct access to the file system, you can right click on the connected device, select Properties, then the Tools Tab, then select Check Volume For Errors to repair the file system.
The same thing can be accomplished by formatting the device (in either mode) and retransferring your books and music. Note that formatting dies erase all media from the device, so everything will have to be retransferred.
Remember, for any protected media with DRM, the Sansa has to be switched back to MTP mode,by going to Settings > System Settings > USB Mode > MTP on the device.