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View Full Version : SD Card reliability (again)



Heater.
12-20-2011, 01:56 PM
Over time I have heard a lot said about the reliability of SD cards. About wearing out the flash chips about the compensating wear leveling with the cards, about how we should not worry so much...

BUT here I am with a pile of new but unusable micro SD cards and a seemingly impossible task. All I want to do is clone, that is block for block copy, one 4GB SD card to one or more others. The source SD card contains two partitions, one FAT and one Linux ext3 but that should have no bearing on the issue. My approach to cloning is to use the "dd" command under Linux.

So first:
$ dd if=/dev/sdd of=card.img

Then check the copy is good with:
$ md5sum /dev/sdd card.img

OK, so replace the card reader with an empty one and write to it:
$ dd if=card.img of=/dev/sdd

And check the card is a good copy:
$ md5sum /dev/sdd card.img

FAIL!!!

Just in case try to boot my target system with the new copy, FAIL.

For now my original still boots a working target but I get more and more nervous as I can't back it up and restore it.

After a few goes around this loop with new Kingston and SanDisc cards I wonder if it is possible to ever find one that works.
Or am I doing something wrong? This method always used to work for compact flash cards.

Gadgetman
12-20-2011, 02:03 PM
Are you absolutely certain that you have GOOD cards, and not hacked Chinese crap?

Verify that they can actually take the correct amount of data by copying in large files...

Tor
12-20-2011, 02:19 PM
@Heater:
Go to http://oss.digirati.com.br/f3/
download and compile. Works under Linux. This tool (write first, then read) will check if your cards are good or fake. And it does a read+write+checksum test, just as you're trying to do, so should be very relevant.

-Tor

Heater.
12-20-2011, 03:47 PM
Gadgetman,

No not sure at all. Came from a reputable store. But now a days even they can't be sure.

Tor,

Great, never heard of f3 before, it's doing it s thing here now. May take a while...