[lucid] how to mount USB storage with correct permissions?

D. R. Evans doc.evans at gmail.com
Fri Oct 29 00:57:03 BST 2010


Reinhold Rumberger said the following at 10/27/2010 11:42 PM :

>>
>> But the drives are marked as being owned by root, so I can't write
>> to them.
> 
> Actually, that's not true - they should be owned by user root and 
> group disk. Then, any user in the disk group can write to them. 

I'm sorry, but it *is* true:

lrwxrwxrwx  1 root root    4 2010-10-27 13:50 usb -> usb0
drwxr-xr-x  2 root root 4096 2010-10-27 13:50 usb0
drwxr-xr-x  2 root root 4096 2010-10-27 13:50 usb1
drwxr-xr-x  2 root root 4096 2010-10-27 13:50 usb2
drwxr-xr-x  2 root root 4096 2010-10-27 13:50 usb3
drwxr-xr-x  2 root root 4096 2010-10-27 13:50 usb4
drwxr-xr-x  2 root root 4096 2010-10-27 13:50 usb5
drwxr-xr-x  2 root root 4096 2010-10-27 13:50 usb6
drwxr-xr-x  2 root root 4096 2010-10-27 13:50 usb7

> 
>> I tried changing the permissions on /dev/usb and /dev/usb0,
> 
> What are those two supposed to be? They don't even exist here.
> 

Well, they exist here, and that's what Kubuntu calls my drives (don't ask
me why; I have no idea where it gets those names from). When I put a USB
stick into the socket, the device notifier tells me that there's a new
device in /media/usb. And, as I show above, that device is owned by root.
And in particular only root can write to it, which is exactly the behaviour
I observe (and can't change).

>> but as
>> soon as a drive is actually mounted, the permissions change back
>> so that only root is allowed to write to the drive.
> 
> Are they, perhaps, links to the actual device files? If so, the 
> permissions of the links are without consequence.

I don't know what the actual device files are. Kubuntu doesn't mention any
/dev file, just the /media files. Somewhere there must be some kind of
mapping between the actual devices and the /media files, but I have no idea
where that's located; anyway, the desktop seems only to mention the /media
files so that's what I looked at.

> 
>> What do I have to do so that when I insert a USB drive into a USB
>> port I can, as an ordinary user, both read and write to the
>> drive.
> 
> Usually just make sure the user is in the disk group. (In the user 
> management module in system settings, make sure the user has the 
> "Access external storage devices automatically" privileges checked.)

That is checked.

FWIW, this all worked perfectly in every release all the way back to
dapper, without my having to do anything at all. It's so frustrating when
things that have worked for years are suddenly broken :-(

  Doc

-- 
Web:  http://www.sff.net/people/N7DR

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