Linux - couldn't mount RDWR because of unsupported optional features (400)

The ext4 feature (400) is the new metadata_csum feature. If this feature is enabled and old tools are used to mount the filesystem they will only be able to mount read-only.

https://ext4.wiki.kernel.org/index.php/Ext4_Metadata_Checksums

To create a ext4 filesystem without this feature:

sudo mke2fs /dev/sdb1 -O ^metadata_csum

Or turn it off on an already created filesystem:

sudo tune2fs -O ^metadata_csum /dev/sdb1

The error "EXT4-fs : couldn't mount RDWR because of unsupported optional features (400)" is due to different versions between the partition formatter (mkfs.ext4) and the mounter.

You have two options:

a) Either you have to upgrade the mounter program using a newer distro inside the SD-card.

b) or you have to backup the files, reformat the SD-card with the same distro (the same ext4 versions) you are doing the mounting, and after the reformat copy the files again to the SD-card.

In the second option, care must be taken with the original ext4 options the formatter has put, trying to consider the same options at reformat. Note also that a reformat of partitions doesn't need a repartition of the whole device, so the boot MBR would not be altered.