Problem uploading Sketch to Raspberry Pico RP2040 in Arduino IDE – “Unable to build drive list”

So you bought a Raspberry Pico, hoping to upgrade your Arduino workflow to 32 bit, but you ran into problems almost immediately? Join the club…

When I tried to upload a simple sketch, I got an error message “Unable to build drive list”. The cause of this is simple, but fairly deeply rooted.

Short answer:

Cause: Your windows PATH has become corrupted, and is missing key references. This causes the compilation tools to fail as a result

Solution:

1. Open the Environment Variables utility.
2. In the System variables section, edit the PATH.
3. Add the following individual lines back to the Path variable:
   %SystemRoot%\system32\WBEM
C:\Windows\system32\WBEM
C:\Windows\system32
C:\Windows
C:\Windows\System32\WindowsPowerShell\v1.0\
C:\Program Files\PowerShell\7\
C:\Windows\System32\OpenSSH\
%UserProfile%\AppData\Local\Microsoft\WindowsApps

   Note: If any of those are already there, don’t duplicate them.

4. Now, save your changes, and close the Environment Variables utility.
5. Close the Arduino IDE and relaunch.
6. Try uploading your sketch again

Longer Answer:

The error that I was getting in the Arduino IDE was fairly obtuse: “Unable to build drive list”.

After a lot of digging, I narrowed this down. The python script that runs is in ~\AppData\Local\Arduino15\packages\rp2040\hardware\rp2040\2.5.0\tools\uf2conv.py

The error message above is caused by the failure of a command in that script. If you remove the exception handling you see the real error:

File "D:\obj\Windows-Release\37win32_Release\msi_python\zip_win32\subprocess.py", line 395, in check_output
File "D:\obj\Windows-Release\37win32_Release\msi_python\zip_win32\subprocess.py", line 472, in run
File "D:\obj\Windows-Release\37win32_Release\msi_python\zip_win32\subprocess.py", line 775, in init
File "D:\obj\Windows-Release\37win32_Release\msi_python\zip_win32\subprocess.py", line 1178, in _execute_child
FileNotFoundError: [WinError 2] The system cannot find the file specified

This is where things can easily start to get confusing. The debug symbols referencing the D drive are a red herring. Ignore them. However, the final line is correct: the system really cannot find the file!What is actually going on, is that the script uses the python module called “subprocess”, and tries to execute a number of commands. The file that it is unable to find is actually in reference to a call to launch powershell. Normally, powershell should be able to be launched from a command prompt (CMD), simply by typing “powershell” and hitting return. However, when I tried this in a command prompt, this was not working – very strange! What this actually indicated was a much deeper root issue: my Windows Path variable had become corrupted, which causes a series of failures in the Python 3 script, and eventually resulting in that error, when trying to upload a sketch in the Arduino IDE.
This was a very weird problem, and none of the many many forum posts I came across were able to pin down the root cause, hence this post. Hope this helps someone!