On 03/06/2018 04:20 PM, Tomas Kuchta wrote:
Try to delete the original files first. Then create empty file using
/dev/zero and copy it to the card. I bet that it will be there on the card
and some of your original data will disappear as result.

My guess is that the card controller is deduplicating your /dev/zero blocks
trying to protect the card from writes.

Tomas

On Mar 6, 2018 7:09 PM, "Russell Senior" <russ...@personaltelco.net> wrote:

On Tue, Mar 6, 2018 at 3:04 AM, Russell Senior
<russ...@personaltelco.net> wrote:
On Tue, Mar 6, 2018 at 3:01 AM, Jim Karlock <jjkarl...@gmail.com> wrote:
My initial attempt to google this was unsuccessful (most people point
out the write protect tab, not my problem).

Bad switch on the write protect tab? (The tab operates a tiny switch.)
Nope.

I can turn the switch to lock and it mounts the device read only very
clearly.  The behavior I observe is that it happily writes /dev/zero
over the block device, but then when I read again, the old data is
still present.
For example, if I flip the tab to write protect tab to "Lock", I get this:

# dd if=/dev/zero of=/dev/sdc status=progress bs=1M
dd: failed to open '/dev/sdc': Read-only file system
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|Perhaps using dd if=/dev/urandom |of=/dev/sdc status=progress bs=1M
...just a thought.

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