How to Mount USB Sticks to AppVMs

(Note: In the present context, the term "USB stick" denotes any USB mass storage device. In addition to smaller flash memory sticks, this includes things like USB external hard drives.)

Qubes supports the ability to attach a USB stick (or just one or more of its partitions) to any AppVM easily, no matter which VM actually handles the USB controller. (The USB controller may be assigned on the Devices tab of an AppVM's settings page in Qubes VM Manager or by using the qvm-pci command.)

As of Qubes R2 Beta 3, USB stick mounting has been integrated into the Qubes VM Manager GUI. Simply insert your USB stick, right-click the desired AppVM in the Qubes VM Manager list, click Attach/detach block devices, and select your desired action and device. This, however, only works for the whole device. If you would like to attach individual partitions you must use the command-line tool (shown below). The reason for this is that when attaching a single partition, it used to be that Nautilus file manager would not see it and automatically mount it (see this ticket). This problem, however, seems to be resolved (see this issue comment). If for some reason the device does not appear in nautilus and you still need to attach just a single partition to a device, you will need to mount it manually; the device will show up as /dev/xvdi (or /dev/xvdj if there is already one device attached - if two, /dev/xvdk and so on).

The command-line tool you may use to mount whole USB sticks or their partitions is qvm-block. This tool can be used to assign a USB stick to an AppVM as follows:

Insert your USB stick.
In a dom0 console (running as normal user), list all available block devices:
```
qvm-block -l
```
This will list all available block devices connected to any USB controller in your system, no matter which VM hosts the controller. The name of the VM hosting the USB controller is displayed before the colon in the device name. The string after the colon is the name of the device used within the VM. Like this:

dom0:sdb1 Cruzer () 4GiB

usbVM:sdb1 Disk () 2GiB

Note: If your device is not listed here, you may refresh the list by calling (from the VM to which device is connected):
```
sudo udevadm trigger --action=change
```
Assuming our USB stick is attached to dom0 and is sdb, we attach the device to an AppVM like so:

qvm-block -a personal dom0:sdb

This will attach the device to the AppVM as "/dev/xvdi", if not already taken by another attached device, or "/dev/xvdj" etc.

You may also mount one partition at a time by using the same command with the partition number after sdb.

Warning: when working with single partitions, it is possible to assign the same partition to multiple VMs. For example, you could attach sdb1 to VM1 and then sdb to VM2. It is up to the user not to make this mistake. Xen block device framework currently does not provide an easy way around this. Point 2 of this ticket comment gives details on this.
The USB stick is now attached to the AppVM. If using a default AppVM, you may open Nautilus file manager in the AppVM, and your stick should be visible in the Devices panel on the left.
When you finish using your USB stick, click the eject button or right-click and select Unmount.
In a dom0 console, detach the stick:
```
qvm-block -d <device> <vmname>
```
You may now remove the device.

Warning: Do not remove the device before detaching it from the VM! Otherwise you will not be able to attach it anywhere later. See this ticket for details.

What if I removed the device before detaching it from the VM?

Currently (until this ticket got implemented), if you remove the device before detaching it from the VM, Qubes (more precisely

libvirtd) will think that the device is still attached to the VM and will not allow attaching further devices under the same name. The easiest way to recover from such situation is to reboot the VM to which device was attached. But if this isn't an option, you can manually recover from the situation by following this steps:

Physically connect the device back. You can use any device as long as it will be detected under the same name (for example sdb).
Attach the device manually to the same VM using xl block-attach command. It is important to use the same "frontend" device name (by default xvdi) - you can get it from qvm-block listing:
```
[user@dom0 ~]$ qvm-block
sys-usb:sda DataTraveler_2.0 () 246 MiB (attached to 'testvm' as 'xvdi')
[user@dom0 ~]$ xl block-attach testvm phy:/dev/sda backend=sys-usb xvdi
```
In above example all xl block-attach parameters can be deduced from qvm-block output. In order:
- testvm - name of target VM to which device was attached - listed in brackets by qvm-block command
- phy:/dev/sda - physical path at which device appears in source VM (just after source VM name in qvm-block output)
- backend=sys-usb - name of source VM, can be omitted in case of dom0
- xvdi - "frontend" device name (listed at the end of line in qvm-block output
Now properly detach the device, either using Qubes Manager, or qvm-block -d command.

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How to Mount USB Sticks to AppVMs

What if I removed the device before detaching it from the VM?

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