When pyboard.py is used to mount a block device the mount can be lost. There are various circumstances in which this occurs.

The repro avoids external hardware by using the ramdisk from the docs. On a Pyboard 1.1 the following scripts were installed:

ramdisk.py

class RAMBlockDev:
    def __init__(self, block_size, num_blocks):
        self.block_size = block_size
        self.data = bytearray(block_size * num_blocks)

    def readblocks(self, block_num, buf):
        for i in range(len(buf)):
            buf[i] = self.data[block_num * self.block_size + i]

    def writeblocks(self, block_num, buf):
        for i in range(len(buf)):
            self.data[block_num * self.block_size + i] = buf[i]

    def ioctl(self, op, arg):
        if op == 4: # get number of blocks
            return len(self.data) // self.block_size
        if op == 5: # get block size
            return self.block_size

import os

bdev = RAMBlockDev(512, 50)
os.VfsFat.mkfs(bdev)
os.mount(bdev, '/ramdisk')
print(os.listdir('/'))
print(os.listdir('/ramdisk'))

ramdisk_lst.py

import os
print(os.listdir('/'))

The following CPython scripts were created:

make_ramdisk.py

#! /usr/bin/env python3
# -*- coding: utf-8 -*-

import pyboard
pyb = pyboard.Pyboard('/dev/ttyACM0')
pyb.enter_raw_repl()
print(pyb.exec('import ramdisk'))
pyb.exit_raw_repl()

test_ramdisk.py

#! /usr/bin/env python3
# -*- coding: utf-8 -*-

import pyboard
pyb = pyboard.Pyboard('/dev/ttyACM0')
pyb.enter_raw_repl()
print(pyb.exec('import ramdisk_lst'))
pyb.exit_raw_repl()

In this session the mount disappears killed by test_ramdisk.py:

$ ./make_ramdisk.py 
b"['flash', 'ramdisk']\r\n[]\r\n"
[adminpete@capybara]: /mnt/qnap2/data/Projects/MicroPython/micropython/tools
$ ./test_ramdisk.py 
b"['flash']\r\n"

In this one it persists. It is possible to enter and leave mpremote at will with the mount intact:

$ ./make_ramdisk.py 
b"['flash', 'ramdisk']\r\n[]\r\n"
[adminpete@capybara]: /mnt/qnap2/data/Projects/MicroPython/micropython/tools
$ mpremote
Connected to MicroPython at /dev/ttyACM0
Use Ctrl-] to exit this shell

>>> import os
>>> os.listdir('/')
['flash', 'ramdisk']
>>> 

In this session it disappears, killed by mpremote ls ::

$ ./make_ramdisk.py 
b"['flash', 'ramdisk']\r\n[]\r\n"
[adminpete@capybara]: /mnt/qnap2/data/Projects/MicroPython/micropython/tools
$ mpremote ls :
ls :
         323 boot.py
         497 main.py
        2999 pybcdc.inf
         534 README.txt
       17538 upower.py
        3252 ttest.py
        8634 sdcard.py
        8698 sdcard_mod.py
          65 ramdisk_lst.py
         782 ramdisk.py
[adminpete@capybara]: /mnt/qnap2/data/Projects/MicroPython/micropython/tools
$ mpremote
Connected to MicroPython at /dev/ttyACM0
Use Ctrl-] to exit this shell

>>> import os
>>> os.listdir('/')
['flash']
>>>