Hello Jan,

JP>>> Using external card reader with 2 slots?

HD>> External card reader yes, but NOT using the 2 slots at the same time.
HD>> The source is the internal SDcard slot: /dev/mmcblk0
HD>> The destination is /dev/sda

JP> OK, to make a long story short:
JP> Do Not Do That

JP> When you make copy from a running system to an other card,
JP> then the source changes while the copy is being made.

Yes I know, but in this case, it does not matter.

JP> Links may be changed, log files may be appended, or created,
JP> data may be changed.

If you use the machine intensively, yes.

JP> So the new copy will likely not run, and even if it does it WILL contain
JP> errors and inconsistencies.

Not if you donot use the machine during copying.

JP> The right way is stop the raspi (type poweroff in a terminal perhaps)
JP> and do the copy on an other system.

Last time I was on a ship with a Pi 2B, only one cardreader and nothing else.
There were no other machines.
He was running a corrupted backup from a card his nephew made for him.
Some people donot shut down the Pi after use ;-(.
I only had one new fresh filled card with me,
and he had another card (same type) I made for him in march 2018.
So I put my new card in the internal slot, and his card from March 2018
in the cardreader and we made the backup with the internal SDcard copier
program. This way we always had a working card (from his nephew) if things went
wrong. During the backup we did nothing with the machine and it took 90 minutes
to complete the backup. A long time compared to the 28 minutes I got at my Pi
3B+ with two cardreaders and dd with 1M blocks.
I did not expect a three times longer backup time at a Pi 2B versus a 3B+
But afterwards the new copy worked much better than the crappy backup did.

When sailing and traveling with my foldable bike,
I do not have enough room to transport a large amount of computer hardware.
So next time I have to take 2 cardreaders with me, and leave that clean
underware out of my case ;-).
I did not use it anyway this time.
But if I take two cardreaders with me, you should see I only need one,
and because of an accident (bad meal), I do not have clean underware.
So it is always trouble to pack the right things when traveling ;-).

Henri.

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