https://www.atari-shrine.co.uk/help/con ... disks.html
C'est que j'ai fait pour certaines disquettes.Large MSA Files (Over 720KB)
The process described above involves copying MSA files onto a floppy disk to transfer them onto your ST. Unfortunately, many MSA files are larger than 720KB and thus will not fit onto a single floppy disk. It is possible to split them into 2 smaller MSA files which can be copied onto 2 floppy disks and then reassembled onto 1 target disk on your ST.
First of all, convert your large MSA file into an ST file (ST files are another form of ST disk image used by ST emulators) To do this, run "MSATOST.exe" (click to downloa) from a DOS prompt within windows. You need to specify the name of the source (MSA) file and the name of the new target (ST) file eg:
> MSATOST large.msa large.st
This will create an ST file. Now run "STTOMSA.exe" to convert the ST file back into an MSA file. To create 2 smaller MSA files, simply specify 2 output file names eg:
> STTOMSA large.st smaller1.msa smaller2.msa
This will create 2 MSA files, each of which will be about half the size of the original large MSA file. One MSA file contains the data for tracks 0-40(approx) of the target disk, the other contains data for tracks 41-80(approx).
Now copy these 2 MSA files onto 2 floppy disks and transfer them onto your ST with "MSA.prg" using the process described earlier - REMEMBER TO USE THE SAME TARGET DISK FOR BOTH OF THE SMALLER MSA FILES... YOU SHOULD HAVE ONLY CREATED ONE ST DISK AT THE END OF THE PROCESS!