Unable to remove the files from an usb drive (neither by dd /dev/zero nor by rm -r)

Question

I have a flash usb drive and up till now it has worked well. Recently I recorded iso to it using dd. Now I want to delete it.

$ lsblk
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
.......
sdb      8:16   1  14.6G  0 disk 
└─sdb1   8:17   1  14.5G  0 part /media/alex/ARCH_201404
sr0     11:0    1  1024M  0 rom  

$ mount 

/dev/sdb1 on /media/alex/ARCH_201404 type iso9660    (ro,nosuid,nodev,uid=1000,gid=1000,iocharset=utf8,mode=0400,dmode=0500,uhelper=udisks2)

When I did this

$ sudo dd ibs=4096 count=1 if=/dev/zero of=/dev/sdb1
1+0 records in
8+0 records out
4096 bytes (4.1 kB) copied, 0.00053675 s, 7.6 MB/s

it seemed to succeed but when I explored the usb flash all the files were still there. When did this:

sudo rm -r /media/alex/ARCH_201404/*

and I got the error:

..................
rm: cannot remove ‘/media/alex/ARCH_201404/loader/entries/uefi-shell-v1-x86_64.conf’: Read-only file system
rm: cannot remove ‘/media/alex/ARCH_201404/loader/entries/uefi-shell-v2-x86_64.conf’: Read-only file system
rm: cannot remove ‘/media/alex/ARCH_201404/loader/loader.conf’: Read-only file system
.....................

What can I do about it?

Solution 1

You only zeroed the first 4kb of the partition. Usually all file systems keep a few unused blocks at the start of their partition in order to give space to boot loaders that might be installed on the partition itself. I think that at least 16 blocks are always kept unused.

You copied, with dd, a file system of type ISO 9660, so you have 2048 bytes blocks.

ISO 9660 reserve about 32kb for boot loaders, as explained here: http://wiki.osdev.org/ISO_9660#System_Area

So, in order to really delete the content of the partition, you may need to delete at least the first 1Mb.

Solution 2

You may try remounting the drive with read-write flag. Something like following should work.

# mount -o remount,rw /dev/sdb1 /media/alex/ARCH_201404

Comments

[Mat]

Unmout the drive before you dd anything to it! Now you need to reformat it (after you've unmounted it if you can).

[Avinash Raj]

drive is mounted in readonly mode, please remount it in readwrite mode, so that you can be able to remove files from that.

[eppesuig]

@AvinashRaj the file system cannot be mounted in read/write since its type is iso9660. This file system is always in read only.

[mikeserv]

@Alex - this guy is correct. I didn't realize it, but I guess specified only 4kb when I meant 4mb. With dd you can just use bs=4M or bs=$((4*1024*1024)). Thanks, eppesuig. By the way - this is the reason I suggested you zero out the iso partition table in the first place. They're a pain on usb.

[eppesuig]

This cannot work: iso9660 is a read-only file system type.

[shantanoo]

Yes that correct. One can recreate the iso9660 fs using 'mkisofs' command.