Hello David,

>> 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.

I mean mounting them.

>> 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.

DR> What do you mean by 'read'?

I wanted to mount the new card after making a backup on it,
and that failed because it has the same UUID as the one booted from ;-(.

> Do you mean using dd to /dev/null,

No, not at all.

> or using dd to copy it on the card the machine has booted from?

No, I never did that.

> I'd be very surprised if the former failed,

Yes, but i did not even tried that.
The new card is ok, and you can read it with dd,
but you can NOT mount it any more if it is a 100 % backup from a Raspbian
Stretch Linux you booted from.
Does not matter you made that backup with the Raspbian util SDCard copier,
or using dd with one or two card reader/writers from the local /dev/mmcblk0.
With the SDcard copier you now have the option at Stretch to change the UUID
during copying from the internal microSDcard you booted from, i.e. /dev/mmcblk0
to a card (same make, type and capacity) via a sdcard reader/writer at
the destination: /dev/sda or /dev/sdb etc.

> and you should not be doing the latter,
> as writing to a running image will cause it to be corrupted.

Yes I knew and never did.
With the Raspbian SDCard Copier from the Accessoires menu that is impossable.

> When copying between cards, it is common to find that no two of the same
> stated capacity have exactly the same number of sectors, and you can't
> perform an exact copy from a larger card to a smaller one.

I know for many years.
That often happened when using different makes and/or types,
but I seldom do that.

> To get around this, after setting up an Pi image and allowing it to
> expand to the size of the card, I then remove it and use gparted to
> shrink the last partition by 1M. This means it can safely be copied to a
> slightly smaller card without losing anything vital.

I know this tric and I sometimes did.
But nowadays I allways use the same make, type and capacity of microSDcards.

> Note this may not be possible with the NOOBs card layout,

I know. It has a different layout at the end of the card.

> which is why I avoid it.

Sorry, but I hate NOOBS for more reasons ;-(.
I never use it anymore, I only did:

pi@raspberrypi:~ $ sudo dd if=/dev/sda of=/dev/sdb bs=1M

at a Pi3B booted from a Raspbian Jessie Linux,
but the card on /dev/sda is a Raspbian Stretch Linux.
When booting from that new Stretch Linux,
i.e. in the internal card slot: /dev/mmcblk0
I can NOT mount the backup in a cardreader made from that same Raspbian
Stretch Linux, because it has the same UUID as the booted one ;-(.
That's now the new problem there never was before Stretch, i.e. with Jessie,
and I donot understand why the Stretch developers made that differend UUID
decision?
So now my solution is to made two Stretch versions;
one at a 16 GB card, and one at a 32 GB card, so they are different.
Or mount the Stretch crad from a Pi booted from Jessie.

From the help of de SDcard copier util:

"
Under Raspbian Stretch and later versions, you cannot mount two partition,
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.
"

When backing up using the Raspbian SDcard copier I remarked that the copy had
less free space than the original card from the same make, type and capacity.
I donot understand why? I only use Kingston microSDcards.
When I made the backup using dd with two card readers/writers,
the free space is the same at both cards,
but then you have also the same UUID's,
wich is a conflict when you want to mount them.

Is there a utility to change the UUID on a backed up card afterwards?
Or is that only possible when making a backup with the Raspbian SDCard Copier?

Henri.

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