To start with, your "HDA" file can just be a file full of zeros. You can create it using the Linux dd command on your Pi
pi@rascsi:~ $ dd if=/dev/zero of=/home/pi/new_drive.hda bs=4M count=100
100+0 records in
100+0 records out
419430400 bytes (419 MB, 400 MiB) copied, 22.4022 s, 18.7 MB/s...