I purchased a USB floppy drive on Amazon that supports DS and DD disks for my Ubuntu laptop. First - I format the disk on my 1040 STf then stick the floppy in the USB drive. I use the linux dd command to copy .st floppy images onto the floppy drive. Works well if the .st image is valid:
$ sudo dd if=/path_to_imagefile of=/dev/sda
A second method I have used is to simply mount the formatted floppy on the Ubuntu laptop and copy the files directly to the floppy disk. The key seems to be formatted the disk on my 1040 STf. I have tried many disk VFAT/MSDOS formats from Ubuntu and none seems to work on the Atari but the USB floppy has no issues writing images or files to disks formatted on the Atari.
The third method I have used is Jim Mackenxie's Ghost Link "PC Dosbox" method found in his blog here:https://www.jamesfmackenzie.com/2016/02 ... -pc-to-st/
The comments in the blog also explain how to use a null modem as a serial to TCP IP connector for accessing telnet Atari BBS's from an ST Running the TAZ terminal program - Great Fun!