I eventually found a solution or workaround for this problem. It seems to be to with support for HS400 mode. Removing the two HS400 references in the device tree (rk3399-nanopc-t4.dtb) stopped the errors occurring.

There were hints in my investigation that this occurs with recent linux kernels.

Hi, I having trouble with my new T4 when I come to writing to the built in emmc memory. I'm trying to create partitions and filesystems on it. I get errors like below especially when creating filesystem and/or writing files to it. I managed to get ext4 to appear to work by running fsck after mkfs. H...

I'd like to get the files for 3D printing this housing too.

Thanks