I've just retested it on my M5Stack Fire running UiFlow v1.9.1-fire firmware and I can list the content of the SD card just fine.
Blockly code:

Micropython code:

from m5stack import *
from m5ui import *
from uiflow import *
import os

setScreenColor(0x222222)
print(os.listdir('/sd/'))

Make sure the SD card is inserted before you power the M5Stack Fire as it is mounted while booting up (mounting code is in boot.py). In the log you'll see this:

SD card mounted at "/sd"

Thanks
Felix

sorry to hear. I am afraid I am out of ideas at this point. Sorry.

Thanks
Felix

the following micropython code works for me with a 16GB SD card and running UIFlow firmware 1.7.1.

from m5stack import *
from m5ui import *
from uiflow import *

setScreenColor(0x222222)

label0 = M5TextBox(34, 41, "Text", lcd.FONT_Default, 0xFFFFFF, rotate=0)

with open('/sd/test123.txt', 'w+') as fs:
  fs.write('Hello World')
with open('/sd/test123.txt', 'r') as fs:
  label0.setText(str(fs.read()))

BTW: Please make sure your SD card, when inserted, is completely in and flush with the case and doesn't stick out. I am mentioning that because I also have an M5Stack Gray which has a broken SD card slot in the sense that the SD card, when inserted, still sticks out a little bit and that makes the SD card fail as the pins do not make proper contact. Maybe your M5Stack Fire has a similar issue.

Thanks
Felix

import os
os.mountsd()
os.listdir('/sd')

import uos
uos.sdconfig(uos.SDMODE_SPI,clk=18,mosi=23,miso=19,cs =4)
uos.mountsd()

from m5stack import *
from m5ui import *
from uiflow import *
import os
sd_mount()

from m5stack import *
from m5ui import *
from uiflow import *

setScreenColor(0x222222)

with open('/sd/test.txt', 'w+') as fs:
      fs.write('Hello World')

And it gives me the following error: OSError: [Errno 19] ENODEV