Transfer Atari ST images from PC to Atari ST using floppy disks

There are many very good options to transfer Atari ST images to an actual Atari ST using devices like a HXC Floppy Disk Emulator or an Ultrasatan.

However I wanted to transfer a few games to floppy disk using items I had on hand. This post explains how I achieved this.

I don’t have a PC with an internal floppy drive. I do have a USB floppy disk drive. One of these, which is a TEAC brand. My understanding is that not all USB floppy drives can format a disk to 720KB. Luckily this one can.




Generally .ST game images come in 720KB size and 820KB size. For 720KB images it is possible to image the floppy disk in the USB floppy connected to the PC. However for 820KB images these can not be written using the floppy drive. So first up we can deal with the 720KB images.

Start out by formatting the floppy disk. If you do not have DD floppies, you can use HD floppies but will need to use tape to cover the notch on the top left hand corner. Otherwise the floppy drive will not be able to format the disk at 720KB. Note that when used in the Atari ST the hole does not need to be covered for the drive to recognise the disk.



To format the floppy disk, open a command window and type format a: / T:80  /N:9


Note that you may need to substitute a: for b: depending on what the drive letter assigned to the USB floppy is.

To write the 720KB .ST file to the floppy disk, use a program called OmniFlop

Download and run the software.



Click Next



Click OK if you get this warning


Select Write disk



Select the relevant drive. In my case it was B:




Select the appropriate image format 3.5″ Atari ST DSDD


Browse to your 720KB .ST image file




The image writing process will begin.



Once completed, you will see the success message.

Now for 800KB – 820KB .ST images it is a bit trickier. The external USB floppy cannot handle this format. So what needs to happen is to split the file into 2 pieces on the PC, copy the files onto 2 separate 720KB floppy disks and then write them back together on an actual Atari ST. It is a bit clumsy, but it works, doesn’t cost anything and is worth it if there is a game you really want to play!

The splitting of the file into 2 pieces will also convert the files to .MSA format. For this a program is required named STTOMSA. Extract and place the program into the same folder as your .ST files. Be aware that this program will not work with 64 bit Windows. I used it on an old Windows XP 32 bit laptop.

Open up a command window and change to the directory containing the .ST files and STTOMSA. There is documentation available at 

Use the following syntax to split the files:

STTOMSA <ST filename> <filename1.msa> <filename2.msa>

For example:



After the process is complete, 2 new files named as specified will appear in the folder. These files will not be exactly 50% in size each, but represent a sort of half split. Now copy those 2 files to separate floppy disks.

Now to piece them back together on the Atari ST you can use software named MSA_2_32

Extract the PRG file and place on a disk that can be used in an Atari ST.

Load up the software on the Atari ST, and get another floppy that will be used to extract the image to. This does not need to be formatted in advance.




Refer to the picture above. Up the top, select Diskdrive B. This represents that the output will by made to drive B, you will be asked to put the target disk in when ready to drive A. The reference to drive B: is sort of like a virtual allocation.

Click the MSA file path to specify the source MSA file.

Ensure Format Disk is checked. Do not check Protect Disk.

Click File – Disk when ready. Firstly the contents of the first file will be read, then you will be asked to insert the target disk.

Once you have completed the first part of the disk image, place the disk with the second part of the msa file in the drive, and click on the MSA file path to select the latter MSA file.

Click File – Disk, and the second part of the disk will be read, then formatted and applied to the target disk.

Finally you have the complete image file on the floppy, ready to be used to play on the real ST. Yes it is some effort, but it works.




  1. Hello,
    Juist a question from a noob 🙂
    I tried to fix the msa files wit MSA_2_33 prg.
    When I run my 1040stf the first file goes without problem to disk. When write/format stops I insert second disk with last Msa file.
    And that’s were it ends, program shuts off or Atari goes bonkers!
    What could go wrong? Because first part goes wel.


    1. Hi There,

      Hmm I am not sure. Have you tried splitting a different file in case there is a problem with the source file you have split?

      What file are you trying to do this with?

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