Ok, but the device that the disk is from (80's drum machine) only takes double density disks ie 750kb, although I have used HD disks on it successfully. But in this scenario it should be formatted with the lower density of the DD disks then. I also know nothing about the filesystem used (or sector sizes) I don't really know. I managed to get the block size with this:
And it returns 512, but my guess is that it could be based on the system definition of a floppy.
ioctl( fileDescriptor, DKIOCGETBLOCKSIZE, &blockSize );
Also, a DD disk should atleast have the information spread out more sparsely than the HD disks, same size twice the capacity.