For the first time using Dietpi, I installed it on a Rapi 4. I wanted root on a SSD-card partition (/dev/sda), and all data on another SSD-card partition. As was suggested, when moving /root to /dev/sda1, I started without any installation of any optimized software. No problem (except that btrfs apparently did not work??).
Then, I tried to move userdata to the other SSD-card partition using the drive_manager, but it was impossible because /mnt/dietpi_userdata did not exist yet. Therefore, I installed some optimized software so that /mnt/dietpi_userdata came to exist. Then, I could move userdata to /dev/sda2 after letting drive_manager mount /dev/sda2 as /mnt/. I prefer to mount it on a folder I call /mnt/data. I looked in fstab, but was told to use the drive_manager. I tried my best, but it was apparently impossible to change the mountpoint-name through the Dietpi interface. So, I changed the mount point name and adjusted fstab, and after a couple of reboot all seem to work.
Altogether, this is of course no catastrophy, though it is a bit clumsy, and for some one who is not an experienced linux-user, I think it could easily have been confusing. Well, I wasted a couple of hours on this fumbling around. If I had installed debian, I would probably not have wasted THAT time.
Never mind, I enjoyed installing nextcloud with my head under my arm. It just worked out of the box. Thank you very much, for your nice work with optimized software!!! I hope you can use my account of these small problems.
yep you could move rootFS as well as bootFS to USB/SSD devices
Maybe you can have a look to the following forum post. There are already a couple of descriptions how to transfer from SD to USB devices
https://dietpi.com/forum/t/services-failing-after-swapping-backup-drives/3869/18
https://dietpi.com/forum/t/services-failing-after-swapping-backup-drives/3869/6
https://dietpi.com/forum/t/move-boot-partition-to-smaller-sd-card/3687/7
as well it should be possible to flash DietPi directly on an USB device right from the beginning. But honestly I never tested this 
Has it now become feasible to boot a pi4 from an usb drive without having /boot at the SD-card?
https://www.raspberrypi.org/forums/viewtopic.php?t=261926
See RPi Page
https://www.raspberrypi.org/documentation/hardware/raspberrypi/bootmodes/msd.md
Raspberry Pi 4
The Raspberry Pi 4’s bootcode is stored in EEPROM and can be updated. Support for mass storage boot will be added in a future update.
Known issues
- The default timeout for checking bootable USB devices is 2 seconds. Some flash drives and rotational harddrives power up too slowly. It’s possible to extend this timeout to five seconds (add a new file timeout to the SD card), but there are devices that fail to respond within this period as well.
- Some flash drives have a very specific protocol requirement that is not handled by the bootcode and may thus be incompatible.
- Lack of power can be an issue, so it is recommended to use a powered USB hub, particularly if you are attaching more than one storage device to the Raspberry Pi. If your device has its own power supply, then use that.