USB Drives, dd, performance and No space left

Please note: sudo dd is a very dangerous combination. A little typing error and all your data can be lost!

I like to make copies and backups of disk partitions using dd. USB drives sometimes do not behave very nicely.

In this case I had created a less than 2GB FAT32 partition on a USB memory and made it Lubuntu-bootable, with a 1GB file for saving changes to the live filesystem. The partition table:

It seems I forgot to change the partition to FAT32, but it is formatted with FAT32 and that seems to work fine 😉

$ sudo /sbin/fdisk -l /dev/sdc

Disk /dev/sdc: 4004 MB, 4004511744 bytes
50 heads, 2 sectors/track, 78213 cylinders, total 7821312 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000f3a78

   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1   *        2048     3700000     1848976+  83  Linux

I wanted to make an image of this USB drive that I can write to other USB drives. That is why I made the partition/filesystem significantly below 2GB, so all 2GB USB drives should work. This is how I created the image:

$ sudo dd if=/dev/sdb of=lubuntu.img bs=512 count=37000000

So, now I had a 1.85GB file named lubuntu.img, ready to write back to another USB drive. That was when the problems began:

$ sudo dd if=lubuntu.img of=/dev/sdb
dd: writing to ‘/dev/sdb’: No space left on device
2006177+0 records in
2006176+0 records out
1027162112 bytes (1.0 GB) copied, 24.1811 s, 42.5 MB/s

Very fishy! The write speed (42.5MB/s) is obviously too high, and the USB drive is 4GB, not 1GB. I tried with several (identical) USB drives, same problem. This has never happened to me before.

I changed strategy and made an image of just the partition table, and another image of the partion:

$ sudo dd if=/dev/sdb of=lubuntu.sdb bs=512 count=1
$ sudo dd if=/dev/sdb1 of=lubuntu.sdb1

…and restoring to another drive… first the partition table:

$ sudo dd if=lubuntu.sdb if=/dev/sdb

Then remove and re-insert USB Drive, make sure it does not mount automatically before you proceed with the partition.

$ sudo dd if=lubuntu.sdb1 if=/dev/sdb1 

That worked! However, the write speed to USB drives usually slow down as more data is written (in one chunk, somehow). I have noticed this before with other computers and other USB drives. I guess USB drives have some internal mapping table that does not like big files.

Finally, to measure progress of the dd command, send it the signal:

$ sudo kill -USR1 <PID OF dd PROCESS>

Above behaviour noticed on x86 Ubuntu 13.10.

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