Recommendations for Windows file server

you can use mount points within the VM to break up the contents of the drive while keeping it within the single share / folder structure.

Example:  

Right now, you have:
D:\MyShare
D:\Myshare\Folder1
D:\MyShare\Folder2
D:\MyShare\Folder3

You would TEMPORARILY rename Folder2 to "Folder2A" (or move it out of the MyShare folder to another point outside the share structure but on the same drive (this would allow the data to "move" quickly, just updating file system pointers as opposed to moving it to a different volume which requires a complete system based copy and delete).

Then you take a new VHD and attach it (mount) to a new Folder, "Folder2"

Finally, you copy all the contents of Folder2A into Folder2.

There's an interruption to users as the process occurs, but once done, no one knows (test with a small folder first to see how this would work.

That said, I don't have any hesitation beyond backup and restore / file move times with large VHDX files (you can't use a VHD for > 2TB; you must use VHDX.  If the current file is VHD, you can convert it but if memory serves, you have to take the disk offline to convert.

Sounds like you may be getting to the point where you should consider something like DFS.

Probably the simplest way is to buy some bigger hard disks...create the new RAID...test it and when you are ready just copy paste the VHD...in the mean time purchasing a couple of 4TB commercial HDD to take a complete copy on both of them of the VHD it will cost you less than $200 and you will be covered no matter what.

Lee, what do you mean by "Then you take a new VHD and attach it (mount) to a new Folder, "Folder2""?
How do you attach a VHD file to a folder?

John, I agree that's the simplest solution but (a) I am storing on SSD so enterprise SSDs are significantly more expensive, but the bigger issue is (b) I'm worried about having everything on a single growing volume. I know I am not near any limitations in the OS, but I was asking for perhaps a way to split up the share across multiple volumes.

Robert, DFS sounds like the perfect solution. I'm going to look into this over the weekend.

Thanks everyone for your input!

The same way you attach a drive to a folder - since that's what it is.  Like I said, test first with a demo folder.

The VM OS is going to see another VHD as a physical hard drive.