Problem till exist (RPI2) when you Change your root Partition to usbstick
https://dietpi.com/forum/t/tutorial-outdated-rpi-move-the-filesystem-to-a-usb-drive/317/1
tryed also:
reboot from sdcard, copy again /DietPI/ * (config.txt, dietpi Folder,…) to /mnt/usb_1/DietPI but after reboot, Folder is again empty.