External SSD not accessible after mounting

User Support General
1

Hello everyone,

I use Qubes release 4.2.4

I have a question about mounting external SSD disks to create a Qubes backup from a VM/Qube. Tested with Debian and Disaposal VM.

The SSD is visible in Device Manager and can be assigned to the VM TEST.
sys/usb: sda Extreem 55AE

In the console window of VM TEST
[user@TEST ~]$ uname -a
Linux TEST 6.15.10-1.qubes.fc37.x86_64 #1 SMP PREEMPT_DYNAMIC Tue Aug 19 00:47:23 GMT 2025 x86_64 GNU/Linux

[user@TEST ~]$ pwd
/home/user

I created a directory with the mount point .-backup. The permissions were deliberately set to 777 so that the change can be seen after mounting (It is normal for permissions to change after mounting.).

[user@TEST ~]$ ls -l
total 36
drwxrwxrwx. 2 user user 4096 Oct 20 11:14 backup

As usual, you can see the SSD sda in the USB Manager and then xvdi in the VM.
[user@TEST ~]$ lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS
*xvda 202:0 1 20G 0 disk *
*├─xvda1 202:1 1 200M 0 part *
*├─xvda2 202:2 1 2M 0 part *
└─xvda3 202:3 1 19.8G 0 part /
xvdb 202:16 1 2G 0 disk /usr/local

  •                              /home*

  •                              /rw*

*xvdc 202:32 1 12G 0 disk *
├─xvdc1 202:33 1 1G 0 part [SWAP]
*└─xvdc3 202:35 1 11G 0 part *
*xvdd 202:48 1 705.9M 1 disk *
*xvdi 202:128 1 3.6T 0 disk *
*└─xvdi1 202:129 1 3.6T 0 part *

[user@TEST ~]$ pwd
/home/user
[user@TEST ~]$ ls -l
total 36
drwxr-xr-x. 2 user user 4096 Apr 27 11:05 Desktop
…
drwxrwxrwx. 2 user user 4096 Oct 20 11:14 backup

[user@TEST ~]$ sudo mount /dev/xvdi1 ./backup

[user@TEST ~]$ ls -l
total 1056
drwxr-xr-x. 2 user user 4096 Apr 27 11:05 Desktop
…
drwxr-xr-x. 7 root root 1048576 Oct 20 11:33 backup

So far, so good. I can neither create a file on the mounted disk nor change the permissions with sudo chmod -R 777 . As a result, I cannot create new files as user in the mounted file system. sudo touch tt work in the mounted partition.
QubesBackup probably fails for the same reasons as the users.

Message QubesBackup
ERROR wirting Backup to VM TEST failed: dd failed to open /home/user/backup/qubes*
Permission denied.

After rebooting the computer, VM, and several mounting attempts, am I overlooking something essential? I would appreciate any tips.

BTW: The disk can be mounted normally under Windows, and I can also add something to it.

2

hm… you give permission 777 to .-backup but then mount to .backup?

3

Yes, just to show that the permission is there in the directory and that the mount point is working. I understand that chmod -R must come after mounting. However, as described, this no longer works after mounting.

It is a 3TB SSD from Sandisk with extFAT filesystem. Perhaps it is still important to mention.
And then it dawns on me…
extFAT does not support linux-style permissions or ownership. So we need help…

sudo mount -t exfat -o rw,uid=1000,gid=1000,umask=0000,dmask=0000,fmask=0000 /dev/xvdi /home/user/backup

ls -l show now user:user as owner and everything works.

Thanks KitsuneNoBaka for the reply.
Ultimately, it was a simple Linux problem.

4

I don’t remember exactly but if you mount partition without linux style permissions (windows fs’s) then it will mount with user that invoke mount. If you run it with sudo then it will be root.
To mitigate that you need to give mount user/group mount options.