Mount exFAT file system as user without specifying /dev/sdX1 in /etc/fstab

filesystemsremovable-storagestorageusb

I've "solved" the following problem, but not in a desirable way.

First, mounting exFAT file systems in Linux was truly as simple as this site says.

The catch is, "as long as you are root."

A user would like to plug in his exFAT file system anytime, but attempting to do so results in:

mount.exfat /dev/sdc1 /media/exfat-device
ERROR: failed to open `/dev/sdc1'

This works as root, but I was able to solve this so that a user can mount it by adding the following line to fstab:

/dev/sdc1 /media/exfat-device exfat-fuse defaults 0 0

However, this is not desirable because it hardcodes sdc1 which assumes (incorrectly) that everything detected as sdc1 will always be exFAT. As an example I connected another non-exFAT drive which gives me no problems, and it's identified as sdc. Then I connect my exFAT device, which becomes sdd, making the fstab addition worthless.

What do I need to do, without giving the user any sudo privileges or elevated access whatsoever, so that any user can connect an exFAT device to that particular machine and read/write to it?

Best Answer

Most distributions have a command called by blkid. blkid will give you a unique identifier for each drive attached to you linux box. Fstab can uses this identifier, replace the /dev/sdc1 with UUID=XXXXXXXXXXXXX. This means that regardless of the user-space designation (e.g. sdc1 scb2 et cetera) your OS will mount it correctly.

1. blkid

Works whether device is mounted or not.

$ blkid
/dev/sda1: UUID="XXXX" TYPE="ext4" 
/dev/sda2: UUID="XXXX" TYPE="LVM2_member"

2. mount

This is only useful once the block device has been mounted.

$ sudo mount | grep /dev/sda1
/dev/sda1 on /boot type ext4 (rw,relatime,seclabel,data=ordered)

3. lsblk

Shows drive topologies, but not the filesystem types on the devices.

$ sudo lsblk -a
NAME                      MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda                         8:0    0 465.8G  0 disk 
├─sda1                      8:1    0   500M  0 part /boot
└─sda2                      8:2    0 465.3G  0 part

tune2fs

$ tune2fs -l /dev/sda1 | grep magic
Filesystem magic number:  0xEF53

Shows the magic number associated with the device, you can look these up on this site, Linux Magic Numbers. It's also in a file, often here, /usr/share/magic. You can locate it using locate /magic.

dumpe2fs

$ sudo dumpe2fs /dev/sda1 | grep magic
dumpe2fs 1.42.7 (21-Jan-2013)
Filesystem magic number:  0xEF53

Same things apply as in tune2fs.

/dev/mounts

This is the "file" maintained by the kernel that's used to display devices that have been mounted. NOTE: many of the tools on this list are typically using the contents of this file.

$ sudo cat /proc/mounts | grep /dev/sda1
/dev/sda1 /boot ext4 rw,seclabel,relatime,data=ordered 0 0

file

You can also use the file command to display info about unmounted filesystems.

$ sudo file -s /dev/sda1
/dev/sda1: Linux rev 1.0 ext4 filesystem data, UUID=XXXX (needs journal recovery) (extents) (huge files)

An alternative syntax (as root):

$ file - </dev/sda1
/dev/stdin: Linux rev 1.0 ext4 filesystem data, UUID=XXXX (needs journal recovery) (extents) (huge files)

Your mounting issue

I'd suspect that the filesystem you're attempting to mount is not what you think it is. Either the entire device was formatted with a filesystem (/dev/sda), in which case you'd be mounting the entire drive. You can test this hypothesis like this:

$ sudo mount /dev/sda /mnt

Of if you need to explicitly tell what filesystem to use:

$ sudo mount -o ext2 /dev/sda /mnt

NOTE: This is generally not the case that an entire filesystem is laid down on the entire device, /dev/sda, rather they're partitioned into /dev/sda1, etc.

Or perhaps the drive has been partitioned. In this case you'd see these partitions with the above command lsblk, where they'd show up as /dev/sda1, /dev/sda2, etc. If that's the case for you then you'll need to mount the partition instead like so:

$ sudo mount /dev/sda1 /mnt

Again pay special attention to the formatting used on the filesystem of the device, you'll occasionally have to specify it literally to mount.

Since your output from fdisk shows that you have /dev/sda1 I'd be inclined to think that you have your filesystem on the 1st partition.

Device Boot      Start         End      Blocks   Id  System
/dev/sda1              63  1953520064   976760001   83  Linux

So mounting it like this should do the trick:

$ sudo mount -o ext2 /dev/sda1 /mnt

/etc/fstab

If the above mounting commands work then you can add this to your /etc/fstab file.

/dev/sda1 /media/HDD ext2 defaults 0 2