How to transfer files from my iPhone to USB Disk on my Apple Watch?

To transfer files to USB Disk on an Apple Watch:

Files will keep on transferring even if USB Disk is not visible on your Apple Watch. However, transfers are faster when USB Disk is visible. As long as your iPhone and Apple Watch can reach each other over Bluetooth, transfers are done over Bluetooth. This is (very) slow. In the paid version, you can configure some cloud services in USB Disk on your iPhone and later access them directly from your Apple Watch. When your Apple Watch cannot reach your iPhone over Bluetooth, all transfers between the cloud service and USB Disk on your Apple Watch will be over WiFi (this is much faster than Bluetooth). So you can temporarily disable Bluetooth on your iPhone in order to speed up transfers.

If you are transferring videos to your Apple Watch, note that USB Disk offers to convert some type of videos (mov and m4v files) to optimize them for the screen of the watch. This greatly reduces their size and speeds up transfers. When pasting video files in the "Apple Watch" location, a prompt will ask you if you want to convert them.