Hello Jan,

JP> You are copying on a raspi???

Yes, why not, I do not have another Linux machine.

JP> Using external card reader with 2 slots?

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

JP> What is your hardware setup?

Raspberry Pi, 1B, 1B+, 2B, 3B and 3B+, Logitech wireless keyboard and mouse
using a 2.4 GHz USB2A dongle, DVI-D monitor, and one or two
SDcardReaders/writers booted from the latest Raspbian Stretch 2018-06-27
and of course updated and upgraded at 2018-07-22.

JP> I copy as folows.
JP> power off raspi
JP> take card out
JP> put it in laptop

I donot have a laptop with Unix/Linux. but only a Win2K one,
or a desktop with Win XP I hardly use.
But I can boot from a Lubuntu 14.04 CD.

JP> copy to /dev/zero (for demo card speed)

Why should I do that?
It only takes my valuable time ;-(.

JP> that is always about the same as what hdparm -t /dev/card says,

Raspbian does not know that command: hdparm.

JP> no matter what blocksize.
JP> At least on my 8GB card.

You have Raspbian Stretch Linux from 2018?

JP> If you copy to a file on harddisk, say some.img
JP> then you also need to look at how fast that harddisk goes sector by
JP> sector.
JP> USB card readers may do things to speed, especially old ones,
JP> if you saturate the USB2...?

I see.

JP> Here some test on my laptop with ever increasing filesize,
JP> card in /dev/sdc  hardisk partition is /dev/sda5  destination file q1
JP> card is latest 32 GB Samsung cllass 10 or sometging that I mentioned in
JP> this thread before.
JP> do not have more free space ATM, but no real differences:

My problem is the new Raspbian Stretch UUIDs !

JP> So, I do not know what all your problems are, but this is get back to
JP> basics...

That only happens when I am backing up between two differend speeds SDCards.
I mostly use the same make, type, speed and capacity cards.

JP> Of course you should not do the copy on a rapi itself, it is not a real
JP> computah ;-) ;-) ;-)

Besides my old M$ DOS BBS PC and Acorn RiscPC at RISC OS 4.04, I do not have
anything else than the menioned 5 Pi's, Win2K laptop and a Win XP desktop.

JP> And it is just data, sector by sector, does not matter what is on it.

For copying yes, but for mounting, it makes a huge difference.
You can not mount SDcards anymore with the same UUID as the booted one
since Raspbian Stretch Linux, witch is a quirc from March 2018.
And I still do not know why they made that impossable.
With the older Raspbian Jessie or Wheezy mounting a 100 % backup card
was no problem at all.

Henri.

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