Duplicating a USB Stick with dd

I have a USB stick that I use for fixing friends computers (among other things) that runs Arch Linux. It seems that their most frequent problems are either crashed hard drives or a virus that makes their computer unusable. The quick solution to backing their data up in either case is to boot an external drive and use that OS to copy the data off their drive (assuming you can still get to it that is). Unfortunately, flash memory has a maximum number of times that you can write to it, which I hit a bit quicker than I’d like running an operating system off of a USB stick. As you likely guessed, my USB stick is failing (remarkably I’ve been using it to do this for several years).

Last night whilst (oh yes, whilst) brushing my teeth, I had an epiphany. I realized that instead of re-installing Linux on a new USB stick, I could use dd to duplicate one USB stick onto another. I tried it, and sure enough, it works almost perfectly. I say almost because there was one minor problem that I will get to in a minute. Firstly though… The command dd is used for making bit-for-bit duplicates or data. In this case, we’re duplicating the exact bits on one device (a USB stick) to another device (another USB stick). You can actually use dd to duplicate most Linux ISO installation files onto a USB stick as well. It works very similarly to burning a CD. Now that that’s explained, here’s the command I used.

Assuming my source USB stick is at /dev/sdb and my destination stick is at /dev/sdc (partitions don’t matter here because we’re duplication the entire drive, not just one partition).

dd if=/dev/sda of=/dev/sdb

(The if is the "input file" and of is the "output file") That will likely take a while. For me it ran at 2.3 megabytes per second. Yours might run a bit slower as my destination USB stick has an average write speed of 25 MBps which is a bit higher than typical USB sticks (many thanks to Newegg for the great price). On the flip side, my source usb stick was starting to fail, so it might go way faster. I’m not really sure.

Okay, now that one issue I had.

Two different 8 gigabyte USB sticks (that’s what I used) are likely not going to have the exact same amount of space on them. If your destination USB stick is even slightly smaller than your source disk, you’ll miss some data due to the disk being full (in the case of dd, even empty space has bits in it that get transferred). This will cause problems for you because the filesystem will say that it starts here and ends there, when the actual partition ends earlier than expected. While this likely won’t cause issues for you other than your machine complaining about it, it could result in some data loss. Either way, the way we get around this issue is really simple. Once you’ve duplicated your USB stick and booted the new one, you should see the following error:

The filesystem size (according to the superblock) is derpderpderp blocks The
physical size of the device is blerdibler blocks
Either the superblock or the partition table is likely to be corrupt!

Just run this command and it should solve your issue

resize2fs /dev/sd-lastpartition

In my case that command was resize2fs /dev/sdb4 (my /home partition).

May there be utility USB sticks for all!

Category:Linux