linux - dd aplied to dev USB before connect corrupt /dev/sdX and dont work

I have a USB where I'm coping an ISO of 5 Mb aprox with a small OS with its own grub. If I connect the device and check it sudo fdisk -l I get:

...
Device     Boot Start   End Sectors  Size Id Type
/dev/sdb1  *        1  9551    9551  4.7M cd unknown

If I copy with dd the ISO sudo dd if=my_os.iso of=/dev/sdb

14852+0 records in
14852+0 records out
7604224 bytes (7.6 MB) copied, 0.538487 s, 14.1 MB/s

It works perfectly. An boot from BIOS in other PC.

The problem is, sometime I forget to connect the USB after update the BIOS, so I do ff WITHOUT the USB connected, the output is:

14852+0 records in
14852+0 records out
7604224 bytes (7.6 MB) copied, 0.00987684 s, 770 MB/s

Now, the /dev/sdb register is corrupted, the copy speed (770 MB/s) have no sense, and cannot user again dd until reboot.

I have tried to repair it with ddrescue and it doesn't work

sudo ddrescue /dev/zero /dev/sdb conv=noerror,sync

My question is, what is happening exactly and if is possible, how to repair /dev/sdb to avoid reboot and dd start working again?

Thanks.

Answer

Device nodes like /dev/sd* only show up in /dev after the device is connected, not before. (Also, they aren't called "registers".) So when you ran dd of=/dev/sdb it didn't actually touch any device – instead, since the output file didn't exist, dd created a new file by that name.

$ sudo dd if=archlinux-2016.04.01-dual.iso of=/dev/sdc
1482752+0 records in
1482752+0 records out
759169024 bytes (759 MB, 724 MiB) copied, 0.941926 s, 806 MB/s

$ ls -l /dev/sd*
brw-rw---- 1 root disk 8,  0 Jun 15 08:54 /dev/sda
brw-rw---- 1 root disk 8, 16 Jun 15 14:14 /dev/sdb
-rw-r--r-- 1 root root  724M Jun 15 15:06 /dev/sdc

So there's really nothing that could have been "corrupted", and nothing that makes sense to "repair" or ddrescue. You simply have a huge file named /dev/sdb, and because it exists, the kernel can't create a real device node there.

(The write speed makes perfect sense – /dev is stored in RAM, so you just copied the image from RAM, where it was cached, back to RAM, where /dev lives.)

Delete /dev/sdb and reconnect the device, and the real device node should reappear.