Same fs type and fstab line gives different automount permissions for different disks

chadwick · 08-03-2011, 08:45 PM

I have two usb drives. Both are Western Digital. One is maybe a couple years older than the other. The older uses USB 2 and is 300 GB while the newer one uses USB 3 and is 500 GB.

The thing I'm trying to understand is why they would automount with different permissions and ownerships. They are both formatted as a single partition with ext3fs. fdisk -l says:

Code:

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               1       60798   488352768   83  Linux

for the newer one and

Code:

   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1               1       38914   312569856   83  Linux

for the older one.

Aside from the UUID, the fstab entries for the two are identical:

Code:

UUID=91ea837a-e675-4989-bc8c-0efeff31486a       /media/disk1        auto            noauto,user,noexec              0       0
UUID=936bcef4-7817-4911-a9e4-8f1cdc257a1b       /media/disk2           auto            noauto,user,noexec              0       0

where disk1 is the older disk.

Without neither one mounted, ls /media/* -ld reports:

Code:

drwxr-xr-x 2 root root 4096 Jan 16  2011 /media/disk1/
drwxr-xr-x 2 root root 4096 Aug  3 21:03 /media/disk2/

Then if I plug them in it says:

Code:

drwxr-xr-x 33 fred fred 4096 Aug  2 17:27 /media/disk1/
drwxrwxrwx  4 root root 4096 Aug  3 15:05 /media/disk2/

If I unmount them and mount them both with the mount command with username fred then the same thing happens.

If I switch the directories in the fstab file to:

Code:

UUID=91ea837a-e675-4989-bc8c-0efeff31486a  	/media/disk2		auto    	noauto,user,noexec     		0       0
UUID=936bcef4-7817-4911-a9e4-8f1cdc257a1b  	/media/disk1		auto    	noauto,user,noexec     		0       0

i.e., just reverse the names of the directories, then the same thing happens. The difference in permissions still exists, and with the directory names reversed,. So it seems that the permissions and user are dependent on what kind of disk it is:

Code:

drwxr-xr-x 33 fred fred 4096 Aug  2 17:27 /media/disk2
drwxrwxrwx  4 root root 4096 Aug  3 15:05 /media/disk1

I'm curious if anyone can explain what's going on and what the better way to go about configuring the mounting of these disks is.

qlue · 08-03-2011, 09:55 PM

I don't know how to check this exactly, but you need to check the ownership of the drives! The one that is mounting correctly is owned by the logged in user when you mount it while the one that is mounting with read-only is not. (probably owned by root)
I've had this with flash drives and sd cards, it's a common problem after installing a new system. (or using ext2/3/4 file systems between different computers!)

chadwick · 08-04-2011, 09:55 AM

Thanks for pointing that out. It turns out I just had to use chown and chmod to change the ownership and permissions of the directory that gets mounted. Funny that here I thought it was something unpredictable and mysterious about the automatic mounting process.

chadwick · 08-04-2011, 10:21 AM

I guess my confusion arose from the fact that it's not really a directory that gets mounted but a partition. The directory itself stays on the local machine but shows different permissions depending on the partition that you mount on it. It makes sense then that a Linux partition would be designed to be mounted at a directory so would have all of these settings that a directory has and that you would change them by applying them to the directory that it's mounted at.