Ok - I do not know how much help this will be because I am reaching back into the cobwebs of my mind here!! Hopefully someone with a D1 will chime in here!!
I was working for Viking (2006/2007) when they changed the D1 to usb from a disk. I can not remember the format of the usb and remember at that time there was something odd about the format - it was specific to the D1 only. If you plug the usb into you computer can you see the actual file extensions? ie - .vp3 , .hus? If so, then any available software that will read that format will be fine. There was a 4D (5D and 6D) embroidery disk called "Organizer" that came with some of those machines. They also sold bigger Embroidery Software packages but that package
requireda dongle and will not work without that dongle.
Embird (another embroidery software program) had a tutorial that I just saw about writing to a D1 usb but you would need to purchase the program to do that.
I also think if you put your embroidery unit actually on your machine and plug in the usb into the sewing machine, you may be able to "see" the designs through the machine.
If all of that fails, I would suggest trying to join a Viking Sewing machine group (Facebook??) or see if there is a Viking dealer that you could call via phone that might help you.
Sorry that I do not have a definitive answer for you but hopefully it helps you gets you in the right direction!!
Edit to add link to Embird article on writing a D1 usb
https://www.embird.net/sw/embird/tut...u/vikdesu1.htm