[SOLVED] Changing ownership/writing to ext4 USB drive

Lysander666 · 02-18-2018, 11:00 AM

I have a 4GB USB drive which I used fdisk to format using ext4. Now I can't write to the drive. I have tried chown and chmod to change permissions but I still get permission denied. To be clear, I used:

chown -R lysander /dev/sdb
chown $lysander:$lysander /dev/sdb

I have tried adding a line to the mount point in fstab but that's not working either. I am sure I am just missing something very basic.

lsblk:

Code:

lysander@lysurfer_viii:~$ lsblk
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda      8:0    0 232.9G  0 disk 
|-sda1   8:1    0    28G  0 part /
|-sda2   8:2    0     1K  0 part 
|-sda5   8:5    0     2G  0 part [SWAP]
`-sda6   8:6    0   203G  0 part /home
sdb      8:16   1   3.8G  0 disk /run/media/lysander/9f73a14b-0397-41de-bcd3-7b4b68897711

ls -ld:

Code:

lysander@lysurfer_viii:~$ ls -ld /dev/sdb
brw-rw---- 1 root plugdev 8, 16 Feb 18 16:45 /dev/sdb

Now, after chown -R lysander /dev/sdb I get:

Code:

bash-4.3# chown -R lysander /dev/sdb
bash-4.3# ls -ld /dev/sdb
brw-rw---- 1 lysander plugdev 8, 16 Feb 18 16:45 /dev/sdb

But I still cannot write.

fstab:

Code:

/dev/sda5        swap             swap        defaults         0   0
/dev/sda1        /                ext4        defaults         1   1
/dev/sda6        /home            ext4        defaults         1   2
#/dev/cdrom      /mnt/cdrom       auto        noauto,owner,ro,comment=x-gvfs-show 0   0
/dev/fd0         /mnt/floppy      auto        noauto,owner     0   0
devpts           /dev/pts         devpts      gid=5,mode=620   0   0
proc             /proc            proc        defaults         0   0
tmpfs            /dev/shm         tmpfs       defaults         0   0
#/dev/sdb         /run/media/lysander auto user,umask=000,utf8,noauto 0 0

As I say, I am probably missing something quite minor but I'm not sure what it is here.

Skaendo · 02-18-2018, 11:37 AM

Have you tried:

Code:

# chown -R lysander:users /run/media/lysander/9f73a14b-0397-41de-bcd3-7b4b68897711

Lysander666 · 02-18-2018, 11:49 AM

I thought it was something minor - Skaendo, I think you gave me the nudge in the direction I needed.

I tried what you suggested earlier but I just got an error saying the directory didn't exist.

Turns out that the terminal window [when I tried earlier] was not wide enough and as a result lsblk was not printing some of the drive characters [i.e. not just not displaying, but not printing].

e.g.

Code:

lysander@lysurfer_viii:~$ lsblk
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda      8:0    0 232.9G  0 disk 
|-sda1   8:1    0    28G  0 part /
|-sda2   8:2    0     1K  0 part 
|-sda5   8:5    0     2G  0 part [SWAP]
`-sda6   8:6    0   203G  0 part /home
sdb      8:16   1   3.8G  0 disk /run/media/lysander/9f73a14b-0397-41de-bcd3-7b4b6
lysander@lysurfer_viii:~$

extending window

Code:

lysander@lysurfer_viii:~$ lsblk
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda      8:0    0 232.9G  0 disk 
|-sda1   8:1    0    28G  0 part /
|-sda2   8:2    0     1K  0 part 
|-sda5   8:5    0     2G  0 part [SWAP]
`-sda6   8:6    0   203G  0 part /home
sdb      8:16   1   3.8G  0 disk /run/media/lysander/9f73a14b-0397-41de-bcd3-7b4b68897711
lysander@lysurfer_viii:~$

I used your command and the drive unlocked. I can now write to it, thank you.

kilgoretrout · 02-18-2018, 11:51 AM

For future reference, with linux filesystems like ext4 you change permissions on the moutpoint not the device file to change access permissions on the filesytem. Skaendo's suggestion should work. If not, it's probably because you formatted the raw block device, sdb and did not create a partition on sdb beforehand. That works OK in some environments but can cause problems in others.

Lysander666 · 02-18-2018, 11:58 AM

Thanks kilgoretrout, you're right, I formatted the raw block and didn't create a partition on it. It worked though, so I'm thankful.

Is the ownership change specific to the Slack install? I'll try to read it from my Debian box which also has the user lysander.

kilgoretrout · 02-18-2018, 12:34 PM

Quote:

Now, since this device is owned by the user lysander, can it be read/written by any other Linux user on another machine or is the ownership change specific to the Slack install? I'll try to read it from my Debian install which also has the user lysander.

You can run into problems going from one distro to anther even if you're using the same username. That's because linux looks at the numerical user id(uid) to determine access permissions. Debian and Ubuntu based distros all assign the first user created a uid of 1000 whereas Redhat, Mageia, Fedora and other historically RH based distros all assign the first user created a uid of 500. I don't know what Slackware does by default but you can find out by running:

Code:

$ id

It's very annoying for disto hoppers because a username may have access in a Debian based distro and the same user will be denied access in an RH based distro because of the differing uids. The easiest way around the problem is to use chmod to give the world rw access instead of chown which will limit access to a given uid. The downside is the security implications of giving world rw access to the drive. To give the world rw access you would run the following with the deivce mounted:

Code:

# chmod 666 <mount point of the device>

Lysander666 · 02-18-2018, 03:35 PM

Thanks kilgoretrout, I'll use chmod if I run into problems. If anyone's interested I used the USB drive with Debian and it works, I can read from and write to it. So this has worked out rather nicely.

Didier Spaier · 02-18-2018, 03:41 PM

To know more:
man usermod
man stat