First post, be gentle guys I give it a try, even there are mostly 0 replays for months ...

I tried to install Kali Live iso persistent, but with the persistent partition on the HD.
I didn't find any information about this, so maybe it's not possible?

I tried the following:

# parted /dev/sda mkpart primary 500gb 550gb
# mkfs.ext3 -L persistence /dev/sda3

# e2label /dev/sda3 persistence
# mkdir -p /mnt/my_pers
# mount /dev/sda3 /mnt/my_pers
# echo "/ union" > /mnt/my_pers/persistence.conf

....... Here problem* arrives. I wanted to complete with:

# umount /dev/sda3

*Problem: permission denied.

I thought i maybe need to edit the /etc/fstab file, but i can't try as it is read only.

Maybe the whole approach is wrong and there is a better way to do it? Or no way?
Any hints for me would be much appreciated Thanks!