Here is troubleshooting information if you're not able to import files into the SPD-1W:
1. Connect the SPD-1W to the computer.
2. When it appears as an external storage device (PC; within the devices with removable storage. MAC; Desktop icon) click on it to view the files.
3. Delete the Master, Click, and .txt files for the locations that you're wanting to transfer new files to. (Note: Deleting the master folder will permanently delete any .wav files contained in that folder from the SPD-1W's memory. Be sure you have copies of any files that you want to keep).
4. Disconnect the SPD-1W from the computer.
5. Cycle the power on the SPD-1W off and then on again, and then reconnect to the computer.
6. Try transferring your files to the desired folders again.
Some .wav files may have data embedded data in them [metadata] or they may not be in the correct format [16 bit/44.1kHz] which will prevent importing and/or playback.
With these files, you'll want to prep them accordingly. You can use iTunes for this:
How to Convert Files for Importing