Sorry, I missed this part of your message:
I also tried switching the disk from the IDE controller to the SCSI controller, but when I did that, dragonfly couldn't see the disk any longer.
I'm not familiar with Hyper-V, but if it emulates several different types of disk controllers it's best to try them all. VirtualBox has about 10 different types; one that worked for me was AHCI.
When virtualizing, heavy load on the host computer may cause dropped packets in some situations. Some OSes / disk drivers are a lot more sensitive to it than others. I've had a lot of problems emulating recent versions of NetBSD and if I run it side by side with another virtualized machine, there are tons of I/O errors due to timeouts or stalled reads. I don't recall having such problems with DragonFly, but if nothing else works and you tend to have high CPU/disk load, it's worth trying to reduce all other activity on the host computer and see whether the timeouts still persist.
