Hello Jan,

RM>> (you can do without the 'bs' blocksize setting,
TM>> but the copy will go a lot more slowly as it'll copy one byte at a time)

JP> Actually 512 bytes at the time,
JP> and that is often the exact sector size of SDcards,
JP> so normally no need to specify a bigger blocksize.

Sorry but of course there is a difference in speed versus blocksize.

JP> Try it, makes no difference whatsoever, except for reporting perhaps.

See my results below.

JP> What does make a difference is fast cards, and a fast card reader.

Yes, that too.

JP> From
JP> man dd
JP> ibs=BYTES
JP>               read up to BYTES bytes at a time (default: 512)
JP> obs=BYTES
JP>               write BYTES bytes at a time (default: 512)

Remark that bs  ibs and obs.
I donot know the different effects of it.
May be next round when I make new versions for the skippers.

See here my results of several days and hours of SDcard filling
with the latest Raspbian Stretch Linux, OpenCPN and inland ECDIS navigation
charts.

pi@raspberrypi:~ $ sudo dd if=/dev/sda of=/dev/sdb bs=1M
14784+0 records in
14784+0 records out
15502147584 bytes (16 GB) copied, 1707.88 s, 9.1 MB/s

1707.88 seconds is about 48 minutes.

Other tries with the same card reader/writers for source and targets
with all 16 GB Kingston microSDcards Class 10 rated max. 80 MB/sec.:

pi@raspberrypi:~ $ sudo dd if=/dev/sda of=/dev/sdb bs=4M
3696+0 records in
3696+0 records out
15502147584 bytes (16 GB) copied, 1709.89 s, 9.1 MB/s

pi@raspberrypi:~ $ sudo dd if=/dev/sda of=/dev/sdb bs=4M
3696+0 records in
3696+0 records out
15502147584 bytes (16 GB) copied, 1940.11 s, 8.0 MB/s

pi@raspberrypi:~ $ sudo dd if=/dev/sda of=/dev/sdb bs=1M
14784+0 records in
14784+0 records out
15502147584 bytes (16 GB) copied, 1682.93 s, 9.2 MB/s

pi@raspberrypi:~ $ sudo dd if=/dev/sda of=/dev/sdb
30277632+0 records in
30277632+0 records out
15502147584 bytes (16 GB) copied, 6219.24 s, 2.5 MB/s
pi@raspberrypi:~ $

103 minutes, that's much more than the first 48.

For the last Kingston SDcard, I had to use no blocksize, i.e. the default one,
because it was a slower 16 GB Class 10 microSDcard rated max. 45 MB/sec.
The source SDcard was a newer Kingston 16 GB Class 10 microSDcard rated
max. 80 MB/sec.
Using a bigger blocksize resulted in a not good filled slower card,
especially the EXT4 partion was not copied right and had to much free space, en
many directories and files missing ;-(.
Even the Raspbian SDcard Copier could not handle this different source and
target speeds, only DD with default blocksize did the right copy trick.
So beware.
And always dismount the source and target media before starting a backup
with the Raspbian SDcard Copier or the DD-command.
Otherwise you will damage the card's contents when dismounting the original
blank fat image on the just copied one, i.e. small fat and large ext4 parts.

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

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

Geetings van Henri.

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