Error M_400 on a brand new MARS Pro

[cliffyk]

The USB drive needs to be partitioned as a primary partition with a Master Boot Record (MBR, and then formatted as plain ol' FAT32--if it cannot be read, and was formerly partitioned you'll need to do a hard delete of the old partition and redo it as a primary with MBR.  On Windows I have found the free versions of either Partition Master or Disk Genius to work well. On Linux I use the command line fdisk utility.

Had to do this to my Elegoo supplied flash drive just yesterday (I booted to Windows and used Disk Genius) because I was too lazy to look up the Linux fdisk options. To minimize the need to do this always use the "eject" or "unmount" option on your computer before removing the flash drive--and shut off the MARS before unplugging the drive (do not power-up the printer with the drive installed)...