on a linux machine

• connect the USB stick/disk
• find the device name

lsblk

• delete all partitions

parted -a optimal /dev/sd?
rm 1
x

• format the disk ext4

mkfs.ext4 -m 0 /dev/sd?

on the Mikrotik router

• insert the usb stick/disk in the usb port.
• log in, start a teminal and display:

/disk/print
Flags: B - BLOCK-DEVICE; M - MOUNTED
Columns: SLOT, MOUNT-POINT, MODEL, SERIAL, INTERFACE
#    SLOT  MOUN  MODEL                      SERIAL                INTERFACE        
0 BM usb1  usb1  Kingston DataTraveler Max  204479A6A95081A50275  USB 3.20 5000Mbps

to create a directory named persistent-volumes place a file there using:

/file
add name="usb1/persistent-volumes/test" contents="starter file"

https://help.mikrotik.com/docs/spaces/ROS/pages/91193346/Disks