正式なコメント
Hey Michiel,
please follow these steps to ensure you have the drive formatted properly and the file is correctly unzipped and placed on the drive:
1) Please open your downloads folder or desktop and delete any of the UPD files located within. Do the same for your USB drive.
2) Empty the trash from your Finder.
3a - Mac) Open the Disk Utility application and format the USB drive you're using for the update (if there's nothing else on it). Select the drive, not the partition (click View > Show All Devices to make sure the drive itself is visible), and click Erase, then select MS-DOS (FAT) as the format, and Master Boot Record as the scheme.
3b - Windows) Use a 3rd party utility to such as the free EaseUS Formatter tool to format your drive to FAT32, also ensure it's set to Master Boot Record.
4) Download the firmware here using a browser other than Safari as it may corrupt the file on download, then unzip the file.
5) Drag-and-drop the UPD file on the USB drive. Do not rename it.
6) Power off the CDJ then press and hold the IN/CUE and RELOOP/EXIT buttons. while powering on the unit; continue to hold them until prompted to insert the USB drive.
7) Insert the USB drive into the USB port when requested and wait until the process has completed.