[Bug] Pybricks Beta - EV3 - "This hub is already in use by another application." - Pop!_OS (Debian)

[sunsaffron]

Describe the bug

When connecting an EV3 brick to Pybricks Beta over USB, the following error is displayed when trying to connect:

"This hub is already in use by another application."

Environment Info:

Pop!_OS (Debian)
Pybricks Beta
EV3

Troubleshooting:

• Tried different USB cables
• Restarted computer
• Tried different USB ports
• Pybricks firmware has been installed on EV3 brick

[laurensvalk]

Is it possible that you have two tabs open with Pybricks?

Each one will try to auto connect.

We should probably clarify the error message if this is the case.

[sunsaffron]

Hello,

Thanks for the prompt reply - greatly appreciated!

Here are some screenshots of the error:

I've only ever tested with one tab of Pybricks open. If it matters from a HW perspective, my laptop is a Lenovo X280.

[BertLindeman]

Hi,

I assume you're using Chromium as your browser.

My system runs Linux Mint rather than POP!_OS, but both are Debian-based, so the following may help.

To see which port the EV3 is using:

• Make sure Chromium is not running.
• Before connecting the EV3, open a terminal and run:

dmesg -w

• Then connect the EV3 via USB.

You should see something similar to:

[Sat Jul 18 10:14:12 2026] usb 2-1.4.3: new high-speed USB device number 14 using ehci-pci
[Sat Jul 18 10:14:12 2026] usb 2-1.4.3: New USB device found, idVendor=0694, idProduct=0005, bcdDevice= 2.10
[Sat Jul 18 10:14:12 2026] usb 2-1.4.3: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[Sat Jul 18 10:14:12 2026] usb 2-1.4.3: Product: LEGO MINDSTORMS EV3 + Pybricks
[Sat Jul 18 10:14:12 2026] usb 2-1.4.3: Manufacturer: LEGO System A/S
[Sat Jul 18 10:14:12 2026] usb 2-1.4.3: SerialNumber: 0016535166E2
[Sat Jul 18 10:14:12 2026] cdc_acm 2-1.4.3:1.0: ttyACM0: USB ACM device

The last line tells you which device was created. In my case it is /dev/ttyACM0.

To see which process is currently using that device, run:

lsof /dev/ttyACM0

For example, if I leave Chrome open, I get:

COMMAND    PID USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
chrome  228867 bert   95u   CHR  166,0      0t0 1144 /dev/ttyACM0

So in my case Chrome is the process holding /dev/ttyACM0.

Could you post the output of both dmesg -w and lsof /dev/ttyACM0? That should tell us what is using the device on your system.

Bert

[sunsaffron]

Hello Bert,

Thanks for the reply. Here's the output as requested:

[ 3386.025866] usb 1-2: new high-speed USB device number 14 using xhci_hcd
[ 3386.149333] usb 1-2: New USB device found, idVendor=0694, idProduct=0005, bcdDevice= 2.10
[ 3386.149353] usb 1-2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 3386.149362] usb 1-2: Product: LEGO MINDSTORMS EV3 + Pybricks
[ 3386.149370] usb 1-2: Manufacturer: LEGO System A/S
[ 3386.149377] usb 1-2: SerialNumber: 001653451AD5
[ 3386.152652] cdc_acm 1-2:1.0: ttyACM0: USB ACM device

esaffron@pop-os:$
esaffron@pop-os:$ lsof /dev/ttyACM0
esaffron@pop-os:~$

The lsof output (or lack thereof) is funny - I had chromium open at the time:

Maybe I'm missing something trivial here?

[BertLindeman]

Maybe I'm missing something trivial here?

Not sure yet.

lsof shows no using task.
Chromium is open and has no USB connection to the EV3 at that moment.

What happens, if you press the USB connect button in pybricks?
I would expect a normal USB connection, now we have seen there is no other task using /dev/ttyACM0.

If you still get "This hub is already...." do another lsof to see what is keeping the port then.

CUrious about t.

[sunsaffron]

Yes this is quite curious - even when pressing the USB connect button I still get the error and lsof shows no processes holding the EV3:

[sunsaffron]

I think I may have solved my problem - I did a little more digging.

Looks like my user account was not a member of the "dialout" group assigned to the device:

esaffron@pop-os:~$ ls -l /dev/ttyACM0
crw-rw---- 1 root dialout 166, 0 Jul 18 07:19 /dev/ttyACM0

I added my user account with the following:

sudo usermod -aG dialout $USER

And now I can connect:

and lsof shows the chrome process:

esaffron@pop-os:~$ lsof /dev/ttyACM0
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
chrome 9251 esaffron 157u CHR 166,0 0t0 1311 /dev/ttyACM0

[BertLindeman]

Hey,

Very good news.

My next question would have been about authorisation.
You already answered the question before I asked.

Happy pybricks time.

Thanks for the feedback.

Bert

PS in discussions like this it is for most readers / supporters better to paste text instead of images of text.
As an example a python program pasted in text can be simple copied and used in pybricks.
That is not possible with an image of the code.

[sunsaffron]

Thanks for the assistance - I will make a note to post command output as text more often in the future. Another thing I'd like to ask: is this the appropriate forum to troubleshoot issues like this, or is there a dedicated discussion page? I understand that what occurred here is more user error rather than any software defect.

[BertLindeman]

Sure.

In pybricks support in the top is a link to discussions
There are 5 categories:

💬
General
Chat about anything and everything here
💡
Ideas
Share ideas for new features
🗳️
Polls
Take a vote from the community
❓
Q&A
Ask the community for help
Answers enabled
🙌
Show and tell
Show off something you've made

So the choice there is rich.

[BertLindeman]

I wonder if the UI could give a difference message for USB port already in use and no auth to use this USB port?

[KWSmit]

Hi, I'm having the same issue since upgrading from pybricks 4.0.0b11 to 4.1.0b2. With version 4.0 everything worked fine in all use cases: working with Chrome, Chromium or pybricksdev. WIth version 4.1 it only works with Chrome. Chromium and pybricksdev cannot connect to EV3 because of 'resource busy'. My user is a member of dialout. lsof /dev/ttyACM0 gives no output.

I'm working with both Fedora and Pop!_OS.

[BertLindeman]

Hi Kees I see something similar.

Attached the EV3 via USB and had Chrome open with beta.pybricks.com.
So auto connected.
test with pybricsdev run usb prog.py -> resource busy.

Disconnected USB via pybricks button.
test with pybricsdev run usb prog.py -> resource busy.

dmesg:

[za jul 18 17:44:22 2026] usb 2-1.4.3: new high-speed USB device number 15 using ehci-pci
[za jul 18 17:44:22 2026] usb 2-1.4.3: New USB device found, idVendor=0694, idProduct=0005, bcdDevice= 2.10
[za jul 18 17:44:22 2026] usb 2-1.4.3: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[za jul 18 17:44:22 2026] usb 2-1.4.3: Product: LEGO MINDSTORMS EV3 + Pybricks
[za jul 18 17:44:22 2026] usb 2-1.4.3: Manufacturer: LEGO System A/S
[za jul 18 17:44:22 2026] usb 2-1.4.3: SerialNumber: 0016535166E2
[za jul 18 17:44:22 2026] cdc_acm 2-1.4.3:1.0: ttyACM0: USB ACM device
[za jul 18 17:45:53 2026] usb 2-1.4.3: reset high-speed USB device number 15 using ehci-pci
[za jul 18 17:45:53 2026] cdc_acm 2-1.4.3:1.0: ttyACM0: USB ACM device
[za jul 18 17:45:53 2026] usb 2-1.4.3: usbfs: interface 0 claimed by cdc_acm while 'pybricksdev' sets config #1
[za jul 18 17:46:14 2026] usb 2-1.4.3: reset high-speed USB device number 15 using ehci-pci
[za jul 18 17:46:14 2026] cdc_acm 2-1.4.3:1.0: ttyACM0: USB ACM device
[za jul 18 17:46:14 2026] usb 2-1.4.3: usbfs: interface 0 claimed by cdc_acm while 'pybricksdev' sets config #1

Some output:

17:48:30 bert@mammoet Linuxmint:~$ ll /dev/ttyACM0
crw-rw---- 1 root dialout 166, 0 18 jul 17:46 /dev/ttyACM0
17:48:36 bert@mammoet Linuxmint:~$ lsof /dev/ttyACM0
COMMAND    PID USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
chrome  228867 bert  208u   CHR  166,0      0t0 1174 /dev/ttyACM0
17:48:45 bert@mammoet Linuxmint:~$ sudo fuser -v /dev/ttyACM0
                     USER        PID ACCESS COMMAND
/dev/ttyACM0:        bert      228867 F.... chrome

So the chrome disconnect does not really completely disconnect?

Stopped chrome and pybricksdev still gets resource busy.

reboot and no chrome used:

 pybricksdev run usb reset_hoist.py
Traceback (most recent call last):
  File "/home/bert/.local/bin/pybricksdev", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/home/bert/.local/lib/python3.12/site-packages/pybricksdev/cli/__init__.py", line 620, in main
    asyncio.run(subparsers.choices[args.tool].tool.run(args))
  File "/usr/lib/python3.12/asyncio/runners.py", line 194, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/asyncio/base_events.py", line 687, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/home/bert/.local/lib/python3.12/site-packages/pybricksdev/cli/__init__.py", line 236, in run
    await hub.connect()
  File "/home/bert/.local/lib/python3.12/site-packages/pybricksdev/connections/pybricks.py", line 303, in connect
    await self._client_connect()
  File "/home/bert/.local/lib/python3.12/site-packages/pybricksdev/connections/pybricks.py", line 951, in _client_connect
    self._device.set_configuration()
  File "/usr/lib/python3/dist-packages/usb/core.py", line 915, in set_configuration
    self._ctx.managed_set_configuration(self, configuration)
  File "/usr/lib/python3/dist-packages/usb/core.py", line 113, in wrapper
    return f(self, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/usb/core.py", line 159, in managed_set_configuration
    self.backend.set_configuration(self.handle, cfg.bConfigurationValue)
  File "/usr/lib/python3/dist-packages/usb/backend/libusb1.py", line 812, in set_configuration
    _check(self.lib.libusb_set_configuration(dev_handle.handle, config_value))
  File "/usr/lib/python3/dist-packages/usb/backend/libusb1.py", line 604, in _check
    raise USBError(_strerror(ret), ret, _libusb_errno[ret])
usb.core.USBError: [Errno 16] Resource busy

pybricksdev version 2.3.0

[KWSmit]

Those are the exact same results as I get. For me this started with the latest pybricks beta update (4.1.0b2). Using pybricksdev 2.3.2