Hello Jan,

HD> This night my 32 GB version is being copied as a backup.
HD> Unluckily I forgot the bs parameter this time,
HD> so it will take a lot more time I think.
HD> But that does not matter when I sleep.
HD> Tomorow I can report the exact result ;-).
HD> May be I should store SDcard images at an external HDD.

So here are the results as promised:

pi@raspberrypi:~ $ sudo dd if=/dev/sda of=/dev/sdb
60456960+0 records in
60456960+0 records out
30953963520 bytes (31 GB) copied, 10383.6 s, 3.0 MB/s
pi@raspberrypi:~ $

10384 seconds is 173 minutes or 2 hours and 53 minutes.
pi@raspberrypi:~ $ sudo dd if=/dev/sda of=/dev/sdb bs=1M
29520+0 records in
29520+0 records out
30953963520 bytes (31 GB) copied, 3811.36 s, 8.1 MB/s

64 minutes for a 32 GB Class 10 card.
A little bit slower as the 16 GB did took about 28 minutes.
May be I should use 2M buffers for a 32 GB card the next time?
You see here that higher up the buffer from default 512 KB block size
up to 1 MB has a big effect in speed!
173 minutes versus 64 for the same amount is almost a 3 times increase!
That's the proof that setting a right blocksize does matters really.

HD> Another difference I remarked was that the Raspbian SDcard Copier left
HD> less free space on the destination card after copying the same sized
HD> source card.
HD> The 16 GB source card had 2.0 GB free,
HD> and the identical make and type target version only 1.9 GB, strange.
HD> When making the backup with the DD-command, they were exactly the same.
HD> But be very carefull when using DD, as it can be very dangerous when
HD> making mistakes in source and destination parameters, or inserting the
HD> cardreaders in a different order. That insertion order influences the
HD> device name,
HD> i.e. which one is /dev/sda and wich one /dev/sdb?

You unluckily did not answer this point.

A new quirk from Rasbian Stretch (necessarry for a Pi 3B+)
is that you cannot read a 100 % backup of a SDCard of the same size.
At older versions of Raspbian (Jessie/Wheezy) this was no problem.
First I thought my SDcards or reader/writer were defective.
Then I remarked I could read a 16 GB backup SDcard when booted from
a 32 GB Raspbian Stretch OS. The other way round also worked.
But I can not read a 100 % 16 GB backup when booted at a 16 GB card.
The same happens to a 100 % 32 GB backup when booted from a 32 GB card.

In the SDcard copier util I saw a new option to tick: New Partition UUIDs.
I think that will make them different from the original booted from.
Yes, I found the answer today in the help text from the SDcard Copier util:

Under Raspbian Stretch and later versions, you cannot mount two partitions with
the same UUID, so you will not be able to mount a cloned SD card when booted
from the disk from which it was cloned. If you need to do this, check the "New
Partition UUIDs" box before copying.

Ok, that's a change we have to live with since 14 March 2018 at the Pi 3B+.
I luckily now know a solution around this new quirk.
I helps a lot if you have at least two USB2 (micro-)SDcard readers/writers.

Even if you donot have a Pi 3B+ but older ones, it is still a good idea to
start using the last raspbian Linux, in this case Stretch over Jessie or
Wheezy.

But I still do not understand why the resulting copy has a lower free space?
That's why I am using now the dd Linux command as showed above,
which results of course in the same UUIDs, but with the same free space.
So good luck in copying.

Greetings van Henri.

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