29

I just got a micro sd card that I plan on using with the Raspberry Pi.

I used the KDE partition manager to delete the original partition on the card and to write a new 12GiB ext4 partition. After doing this, the card still says that 6% of it, 30.02 MiB, is being used.

Basically, I want to completely wipe of the card of all contents so as to prepare it for a new OS. Would a command like shred or dd do the trick or would that destroy the partition as well?

david6
  • 14,449
  • 4
  • 36
  • 46
firephyz96
  • 401
  • 1
  • 5
  • 4
  • I am biased, but if you want full control of partitions, don't use graphical partitioning, use the good old `fdisk`. (Unless you can't because of GPT or other technical restrictions.) – CijcoSistems Aug 15 '14 at 01:20
  • By the way... does RaspberryPi support loading the kernel from ext4 directly? I remember using FAT for the early boot process (kernel, cmdline, SoC configuration and memory split), and then having the root filesystem on another partition (or NFS if you like) – CijcoSistems Aug 15 '14 at 01:25
  • 1
    [Securely wipe disk - archlinux.org] "Wiping a disk is done by writing new data over every single bit." wiki.archlinux.org/index.php/Securely_wipe_disk –  Mar 21 '15 at 06:00

4 Answers4

36

Your guessing about dd is right. You can wipe the SD card out by the following command (let's assume, that your SD card is /dev/sdd):

Do not interrupt this command or it could possibly brick the SD card.

$ sudo dd if=/dev/zero of=/dev/sdd bs=8192

Note: If this command does not complete successfully and you have to abort it, then most likely it is recoverable with disk partition recovery programs covered in other posts.

It can take some time depending on size and speed of SD card. If you are convinced, that CIA would like to recover your files, then overwrite the SD card with urandom instead of zero:

$ sudo dd if=/dev/urandom of=/dev/sdd bs=8192

dd command from above examples will erase whole SD card, leaving it without any partitions, even with no partition table. So you will need to recreate partition on SD card. You can do this by any partitioning tool like cfdisk, parted (my recommendation) or gparted.

And one more thing: be extremely careful when calling dd command. A typo in of= argument value can cause disaster.

david6
  • 14,449
  • 4
  • 36
  • 46
  • 4
    If you are worried about CIA, you should use `/dev/random` and do it like 5 to 10 times... not to mention that you should also burn it in the oven, catch on fire and bury it deep in the ocean :P – CijcoSistems Aug 15 '14 at 01:17
  • 3
    **@CijcoSistems:** *The suggestion to burn, ignite or bury may work. However, the writing random data (5 to 10 times) method will NOT.* – david6 Mar 21 '15 at 05:54
  • 1
    @david6 Could you explain why writing data many times through overwrite may not work? – Denis Dec 26 '15 at 05:11
  • 9
    Modern non-volatile *solid-state* memory uses **wear leveling** to stop repeated read/writes overusing any particular section of the memory. (See: http://searchsolidstatestorage.techtarget.com/definition/wear-leveling) *This means that data writes intending to 'overwrite' may actually be saved elsewhere. It also means that the true memory capacity is several 100 times more than stated, to allow for on-going reallocation.* As noted in my answer, you must invoke the manufacturer's **secure-erase** function, to bypass any 'wear leveling' effects. – david6 Dec 26 '15 at 05:51
  • 1
    I don't think wear leveling is an issue when you are filling the drive with zeros completely. The real issue would be bad sectors dropping off and not being overwritten. – HilarieAK Sep 16 '17 at 23:14
  • 2
    Indeed, filling the device (what dd does) puts wear leveling out of the question. Besides, wear leveling would tend to (not guarantee to!) make a file of 1/10 the capacity written 10 times still erase the entire device, instead of the other way around, because it would try to make sure it is not written to the same cells 10 times. And finally, forget about random and urandom and multiple passes. This is not magnetic media where the mechanical aspect can cause the edges of a track not to be fully erased by overwriting it, a single pass of zeroes is enough. – Luc VdV Apr 12 '18 at 08:07
  • For security data, just don't use Solid state storage: As stated, it is "easy" to recover data even after random writing due to wear levering: Let say you have critical data at sector 1 -> (internal position 42) Now you want to delete it safely, so you overwrite the data with zeros. You override with 0 the sector 1, but now sector 1-> (internal position 64), so your data is still available at 42 and recoverable. – Adrian Maire May 10 '18 at 14:58
  • Why would interrupting dd "possibly brick" the card? If that were the case, I'd say there's a kernel bug. But no, there's no problem with stopping dd in the middle of writing to the card. – crass Sep 02 '21 at 22:51
  • 1
    @david6 The SD specification does not define a "secure-erase". Individual manufacturers may have a non-standard ACMD which does this, but I've never seen this. Do you have any examples of manufacturers doing this? – crass Sep 02 '21 at 22:56
7

What are you trying to achieve?

(A.) Remove all current partitions, to re-use the card:

To erase partitions and/or re-format a (micro) SD card, just use gnome-disk-utility (aka 'Disks').

OR

(B.) Erase any current (or previous) content on the card, so it can NOT be recovered:

To securely erase the card, you need to EITHER physically destroy the card (render into sub-2mm² particles) OR use a utility (from the vendor) to trigger the secure-erase function.

Multiple writes (of random data), or reformatting will NOT necessarily remove the data.

david6
  • 14,449
  • 4
  • 36
  • 46
  • The `sfill` tool can be used for this purpose. See http://superuser.com/questions/319262/what-are-the-techniques-for-securely-erasing-data-in-linux – Jim Garrison Jul 19 '16 at 23:35
  • 1
    **@Jim Garrison**: That does not address wear leveling (see my earlier comment), and re-mapping of data blocks, done by the inbuilt flash-memory controller. That data is still potentially recoverable. – david6 Jul 19 '16 at 23:39
  • 1
    Further reading: https://www.dslreports.com/forum/r27719820-Securedly-erase-memory-cards – david6 Jul 19 '16 at 23:55
5

If you just made a new partition, there shouldn't be any actual files on it. That 30MB is probably just the filesystem itself. The partition needs a few different tables and whatnot to actually keep track of files.

You can double check that there really isn't anything on it by looking for hidden files with ls -alph from the command line or be enabling hidden files in the "View" menu of Nautilus.

Depending on exactly how you invoke the commands, it is likely that using shred or dd would indeed overwrite the partition table and/or filesystem.

TheSchwa
  • 3,820
  • 1
  • 25
  • 38
4

You can shred the files before removing them:

find /media/user/SD32/ -type f -exec shred -v -f -n0 -z -u   {} \;

shred params mean: Verbose, force deletion of non-writable files, don't write random bytes (speeds up), write zeroes (much faster), truncate and delete the file after writing zeros.

This will get rid of data in a way that they wouldn't be recovered by apps like photorec.

See https://unix.stackexchange.com/questions/27027/how-do-i-recursively-shred-an-entire-directory-tree

Ondra Žižka
  • 3,668
  • 5
  • 35
  • 43
  • 1
    Unfortunately this will not work on SD cards. If you overwrite file on them, you are writing to other sectors of the SD. So, to be effective, you should write on entire SD card, as in the answer. – luca76 Mar 04 '19 at 10:39