I've created a bootable Linux system by following LFS 11.0
The boot device itself is on USB, so I have used initrd as well.
All worked fine. Then I've created a backup of the root filesystem from the host using the command
Then I formatted the USB disk, installed the root filesystem from the backup in a host using the command
I installed grub on the USB disk and updated grub.cfg from the backup copy.

When I rebooted with the USB stick, grub loads the kernel, but the kernel could find the root filesystem. It gives the error in the picture error.jpg. Sorry for the pictures as I couldn't find these errors in any of the logs.

However, when entering the 'exit' command in the initial shell, it continues to load the root filesystem correctly, which is shown in post_error.jpg

I did a sanity check on the extracted filesystem, everything seems to be there.

Is anything specific in the root filesystem that should be installed rather than extracted from tar ball?

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Maybe adding rootdelay=15 to the linux line of the grub menuentry.

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It may be slow to recognise the usb stick. Usually when you boot from usb, you add a rootdelay option to the kernel command line to allow for that.

Colorpurple anticipated me!

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Another thought, does /etc/fstab entry for / uuid correct

I use /dev/sdx style in fstab.

But adding rootdelay solves the problem. Thanks for your help.

I am just curious, as to why it wasn't a problem when I build the rootfs initially on USB stick but needed rootdelay only when I created rootfs from backup.

I tested this theory three times. Whenever I've mounted USB stick partition and installed packages directly to the partition, it didn't require rootdelay.
But when I've installed the rootfs by untarring the packages to the mounted USB stick partition, it requires rootdelay.