/Beryllium/mnt should be the mount point. Did that subdirectory exist beforehand?

Is the SD card empty? If not after the storage.mount(), is /Beryllium/mnt/ empty? It should show what is on the SD card.

Did that subdirectory exist beforehand?

Yes.

Is the SD card empty?

Has a single empty file inside.

I was incorrect in saying that /Beryllium/mnt is empty, it has a hidden file.

>>> os.listdir("/Beryllium/mnt")
['test_file.txt']
>>> os.listdir("//Beryllium/mnt")
['.gitkeep']
>>> os.chdir("/Beryllium")
>>> os.listdir("mnt")
['.gitkeep']
>>> 

mount() is clearly trying to mount from working directory, that is really risky when we are talking about relative paths, and leads to issues like this.
I'm sure if I did anything like "../../../../Beryllium/mnt", it would explode.
Checking with logic is also really slow, and buggy.
The simpliest and safest way is:

1. store getcwd().
2. chdir() to target, then to ... If this succeeds, this means target exists, is a folder and we are currently above it.
3. Attempt to mount, the target name being what is after rfind("/").
4. Go back to the starting directory.

If step 2 fails, it should just go back to starting and raise the error.

This feels connected to #8409, the mount point "link" in the root folder does not show up in an os.listdir("./') from the root folder. Also, if from the root folder you enter os.listdir('./Beryllium/mnt') the SD card contents are not displayed, same for os.chdir('/Beryllium') followed by os.listdir('mnt').

Edit: At one point I apparently thought that CircuitPython couldn't mount SD cards to sub folders or perhaps I just blocked doing so in my code because I ran into something like this..

Another symptom is that os.rename('./sd','/newsd') effectively separates the physical mount point from the mounted file system. The contents of the sd card can then been seen by os.listdir('/sd') and the original placeholder file can be seen by os.listdir('/newsd'). Doing this also seems to hide the /sd folder (and SD card contents) from web workflow.

Sorry, I unraveled an even bigger bug.

Adafruit CircuitPython 9.0.0-dirty on 2024-03-19; Seeeduino Wio Terminal with samd51p19
>>> import board, sdcardio, busio, storage
>>> spi = busio.SPI(board.SD_SCK, MOSI=board.SD_MOSI, MISO=board.SD_MISO)
>>> sdcard = sdcardio.SDCard(spi, board.SD_CS)
>>> vfs = storage.VfsFat(sdcard)
>>> import os
>>> os.listdir()
['.fseventsd', '.metadata_never_index', '.Trashes', '.Trash-1000', 'Beryllium', 'code.py', 'boot_out.txt', 'repl.py', 'boot.py', 'settings.toml', 'LOST.DIR', 'Music', 'Podcasts', 'Android', 'Ringtones', 'Alarms', 'Notifications', 'Pictures', 'Movies', 'Download', 'DCIM', 'Documents', 'Audiobooks', 'Recordings', 'lib']
>>> os.chdir("Beryllium")
>>> os.listdir()
['bin', 'dev', 'etc', 'home', 'lib', 'lost+found', 'mnt', 'proc', 'root', 'run', 'sbin', 'srv', 'sys', 'tmp', 'usr', 'var', 'boot']
>>> storage.mount(vfs, "mnt")
>>> os.listdir("mnt")
['.gitkeep'] # not what is on the sd
>>> os.chdir("/")
>>> os.listdir()
['nt', '.fseventsd', '.metadata_never_index', '.Trashes', '.Trash-1000', 'Beryllium', 'code.py', 'boot_out.txt', 'repl.py', 'boot.py', 'settings.toml', 'LOST.DIR', 'Music', 'Podcasts', 'Android', 'Ringtones', 'Alarms', 'Notifications', 'Pictures', 'Movies', 'Download', 'DCIM', 'Documents', 'Audiobooks', 'Recordings', 'lib']
>>>

This original behavior seems to exist in micropython 1.22

MicroPython v1.23.0-preview.352.g50b43fec1 on 2024-05-29; linux [GCC 12.2.0] version
Use Ctrl-D to exit, Ctrl-E for paste mode
>>> import os, vfs
>>> vfs.mount(os.VfsPosix("/tmp"), "/sub/dir")
>>> os.listdir("/")
['sub/dir', 'home', 'nix', 'lib', 'initrd.img', 'etc', 'run', 'libx32', 'sbin', 'usr', 'bin', 'vmlinuz', 'vmlinuz.old', 'bob', 'var', 'boot', 'dev', 'core', 'lib64', 'initrd.img.old', 'srv', 'opt', 'media', 'lib32', 'sys', 'tmp', 'mnt', 'root', 'proc']

Sat down and made a diagram for parsing paths.
I don't wanna imagine doing this in C.

We would need to take any relative paths and convert them to absolute.
That means also handling /path/to/some/weird/../weirder/./place and handling invalid fat32 characters in this parse.
Then from absolute paths we would need to look into the mount table (my brain stores it as a dict {"/absolute/path": vfsobject}), and then do the following mad procedure:

• split the abs path into segments "/path/to/somewhere" -> ["path", "to", "somewhere"].
• Store "/" as our "current location" and set the root vfs object as the "working vfs object".
• Then for every segment:
  • Check if "current location" + next segment ("/" + "path"), exists in the mount table.
  • If it exists, we set that as the "working vfs object" and add the segment into our "current location".
    (Ignoring as to if the folder / file exists in current working vfs object.)
  • If it doesn't, we check the working vfs object's listdir, truncating the path before it.
    (for example, if we had a mount at "/path" and we are checking as to if the "to" segment exists, we will check if "to" exists in the listdir, not "/path/to")
  • If it exists in the vfs object we check if we have more segments to go.
  • If we do, we have to check if the last segment we just checked exists is a file or a folder.

Then this absolute chonker of an C function would have to be used in every os function.

The abs-path function would have to be seperate in order to reuse in areas like the mount function.

Also, it may be worth consider fixing this externally into perhaps a mount_manager module.
It's a lot of edits to the core.