For some reason Im very interested with a image of this SD card 
Teach us how to create such a copy 
Somehow I missed this post...
adb connect ###.###.###.###:55555
adb root
adb shell
dd if=/dev/mmcblk0 of=<usb_mount_point>/backup.img bs=4096
Android uses modified Linux kernel. Some people can't believe in this, but this is
true.
SD card data (image accessible all the time like a regular file) should be either in /dev/block/mmcblk0 or /dev/block/mmcblk1 (RK3399 can handle two MMC memory and this 0/1 depends on the settings in so called Device Tree).
Instead of dd, cp can be used safely.
Finally it should look like this:
adb connect ###.###.###.###:55555
adb root
adb shell
dd if=/dev/block/mmcblk0 of=/mnt/media_rw/...replace this part with usb FS label name.../backup.img
Or in a much easier way (especially for Windows only users) and directly in the file stored on used computer:
adb connect ###.###.###.###:55555
adb root
adb shell
adb pull /dev/block/mmcblk0
EDIT: does the Rigol mount ext4 filesystems? i guess FAT filesystems are not able to create files bigger than 4GByte. so the USB stick must have another filesystem than FAT.
All modern Linux distributions, including Android, uses EXT4 by default. EXT4 is extremely reliable - small damages (like in a dumped image from a running system on it) can be easily fixed by fsck in no time.
When I started using Linux, power loss with huge HDD (disk with spinning plates, not this SSD rubbish), a lot of small files and with EXT3 on it, resulted in 30 minutes of waiting to FSCK finish scanning and repairing. When I used same disk and same data, but I upgraded to EXT4, it was literally seconds, in the worst scenario, about one minute.
Also Android by default mounts /system as a read only. Apps and user data are stored in a separate FS - if any of those will be damaged beyond fixing, it's not a problem.