Hello Jan,

JP> 1) make a backup of your 16 GB SDcard, NOT on the raspi,

Why no Pi ?

JP> but on a Linux PC:
JP> dd if=/dev/X of=cardimage.iso

You forget two important things, i.e. the block size and the Sync !
So the two commands should be at least:

sudo dd if=/sda of=/dev/sdb bs=1M conv=noerr,sync [RETURN]
sudo sync                                         [RETURN]

When using two card readers/writers,
and sda as the source and sdb as the destination.
That depends on the order you inserted the usb SDcard readers.
To be safe:
First insert the source card in a reader, second insert the destination
card in a second reader/writer. That way the source is always sda
and the destination is always sdb in a Pi with 4 USB ports.
You can check that with the accessoiries SDcard copier tool.
When you first insert the destionation SDcard this becomes sda
in stead of the wanted sdb as device name.
So the insertion order is very important!

Greetings from Henri.

---