HD diskettes on MSX

For swapping files taped HD disks are fine... For storing anything that is valuable for me I would take DD disks... and make backup anyway to some more modern media.

They DO work. You can even format on MSX.

but as NYYRIKKI said, data saved by DD drives on HD disks tend to be unstable, so don't trust them to actually store files, just to transfer files.

It is not that black and white... let's put it this way... HD disks work like summer tires work on icy road... I would not recommend driving trough the country with them, but if you only need milk from local store then most likely the trip goes fine... but not always... Naturally the drive it self and the quality of the individual disks have huge impact to experience. It is not that weird that some drive can't format HD disks at all while other drive may keep the data readable even for years.

If you really need DD disks, I can send them to you. I have many on stock. Most are used and there's also a large stock of new (but written to and labeled) PC-Active disks.
No guarantee that they work.

Unfortunately, there are some tasks you most likely will still need a diskette to perform. And I'm not talking about copy-protected disk software: Flashing my Sharksym MMC/SD v2.2 is one of them (there aren't new firmware versions thou). I'm not sure, but I think this is the same case on other modern mass storage devices.