Correct way to fomat an sd card for /overlay use in OpenWrt

I have several options to format the sd card, and it seems to me some of my sd cards are broken as well.

FAT,EXT-4, MBT, GUID, LBA - I am confused.

What is the correct set of options to format a sd card for use as /overlay FS on my Linkit Smart 7688 Target that has a limited RAM and can be extended by a sd card?

I do this to create the /overlay:

#mkfs.ext4 /dev/mmcblk0p1 
#mount /dev/mmcblk0p1 /mnt 
#tar -C /overlay -cvf - . | tar -C /mnt -xf - 
#umount /mnt 
#block detect > /etc/config/fstab 
#vi /etc/config/fstab

Then, the configuration file gets modified to look like this:

        config 'mount'
          option  target  '/overlay'
          option  enabled '1'

That should work, what issue are you experiencing exactly?

./upper[ 69.739107] EXT4-fs error (device mmcblk0p1): ext4_validate_block_bitmap:395: comm tar: bg 16: bad block bitmap checksum

I think it's because I didn't correctly format the sd card. I'm missing a description of requirements how a sd card is expected to be prepaired.
A description how to prepare a sd card maybe with fdisk would be great. Not like the ones found by googling where fdisk key presures are described, but a description what is required like: Format: FAT32, Master Boot Record table, or FAT32, GUID table with LBA or what ever makes sense...

Follow the instruction linked above, this is a tested and working method.

You can skip partitioning and simply format the entire disk.

That message does not look like a user error... I would say the card is broken, but then it would have complained when you formatted it and did the initial copy.