If you have a DAP with a single drive in it such as a FiiO X1, and you have selected the drive twice, (It shows up on both sides of the Dapper slider) then Dapper can give you this error. Similarly if the drive has changed (formatted or changed cards) the same is true.

 

To solve:

1... Unlock the dapper config
2... Re-select the drives any clicking on their icons on each side of the slider.