Seems like nobody replied with a verifiable solution all these years, so here is my simple fix for OMV 6.
OMV probably required root to take ownership of the drive or partition before it can be shared but it doesn't do that properly with some pre-formatted drives even with ext4, so try this if you can with a previously formatted partition if you can afford to erase it. Go to the terminal or SSH:
lsblk
sudo mkfs -t ext4 /dev/sda4lsblk will list the disks and partitions and then choose your correct partition carefully
sudo mkfs will in this case format the sda4 partition with ext4 format.
Now the folders in this part should be shareable if you try sharing to 'everyone' for example. Once you got it to work, change everyone to what users and admin you want to share with.* **
* for some reason, you have to use \\hostname.local if you want to get to the hostname shared drive under Windows 10 file explorer address bar. This has been described by others.
** Once all the partitions which were previously formatted has been newly formatted by the current OMV root and new shares created with the new partitions, Win10 can now recognize \\hostname instead of hostname.local.