Creating an image of an SD card

[Im4Tinker]

How can the u-boot code be written in an unallocated 4mb and why?

As the early computing ages, when the first mass media appeared, there's a location where the system will look into. This is called sector zero, that's really is the first sector of the mass media.
I don't know why the u-boot author preferred to stay out from a partition scheme, but as the name suggest the program is also a boot loader, as they are lilo, syslinux and grub. All of those are taking place on the sector zero, for the very first step. Unless for x86 system that nowadays using the EFI

Anyways, even a BIOS will start to look at the very first block on the internal flash, the even ARM does it, with its chip internal flash. But the ARM has a program on the flash that will tell to proceed to look to the sector zero. This should explain a little how the Rockchip fellows had designed the process.

[Jay Smith]

How can the u-boot code be written in an unallocated 4mb and why?

As the early computing ages, when the first mass media appeared, there's a location where the system will look into. This is called sector zero, that's really is the first sector of the mass media.
I don't know why the u-boot author preferred to stay out from a partition scheme, but as the name suggest the program is also a boot loader, as they are lilo, syslinux and grub. All of those are taking place on the sector zero, for the very first step. Unless for x86 system that nowadays using the EFI

Anyways, even a BIOS will start to look at the very first block on the internal flash, the even ARM does it, with its chip internal flash. But the ARM has a program on the flash that will tell to proceed to look to the sector zero. This should explain a little how the Rockchip fellows had designed the process.

So, if the u-boot is working good, the flash memory will be seen using lsblk or gparted.
If it isn't recognized, then you must use the DD procedure to install u-boot/UMS.
If that doesn't work, most likely the flash memory has gone bad. It has given up the Ghost. It has been bricked!

[GrahamGo]

Thanks for the great information. I have finally finished getting everything working and shipshape on the Tinker board, now its destined for my electric cupboard where it will hopefully generate some money. So making an archive in the next day or so is a must for me. I would hate to have to do all this over again. Although I made notes along the way it just been a bit of a pain. Mainly due to me, because I am not that hot on Linux, though I can admit to learning (and forgetting) quite a bit these last days. Anyway, thanks for the help.

[Im4Tinker]

So, if the u-boot is working good, the flash memory will be seen using lsblk or gparted

If the u-boot is good, the it will start to serve as SCSI driver,  when we plug the TB to a PC.