If you notice your API calls are randomly failing with HRESULT_FROM_WIN32(ERROR_BUSY) / 0x800700AA / -2147024726, then it's likely that the device driver is indeed busy doing something else. It could be that WMP is syncing content to the device, or maybe Explorer needs to refresh its view and is enumerating content on the device.
So what do you do? Simply deal with it.
The fact is that the driver has a choice when an operation is requested while it is busy doing something else. It can either block the contending request or it can return right away with a special error code (in this case ERROR_BUSY).
The current driver model does not advocate a particular choice; so to play it safe, you (as a WPD client app developer) should check if the error was HRESULT_FROM_WIN32(ERROR_BUSY). In that special case, you can randomly try the operation again in a while. Alternatively, you can put up a dialog telling the user that the device is busy doing something else and the user should try again later. Or maybe you can write an asynchronous wrapper around the API which frees you from worrying about all this. Whichever choice you make, just don't treat it as a hard error and present the user with an error dialog since the user won't understand why it broke.
While special-handling ERROR_BUSY may seem to be a hack and inelegant, unfortunately the behavior is here to stay. Later iterations of the driver model may enforce/advocate an alternative behavior, but the backward-compatibility cross will still have to be carried.