Storage Spaces woes

JacobSteele 

Try "-MediaType HDD" to specify the HDDs.
Explanation: If the SSDs and HDDs are of different sizes the number of SSDs are used as reference, in that case "three".
If you want to use storage tiering you have to use "New-StorageTier" bevore "New-VirtualDisk".
If your DON'T want to use storage tiering I recommend two pools, one with the HDDs one with the SSDs.
For example:
New-StoragePool -FriendlyName HDD-Pool -StorageSubSystemFriendlyName "Windows Storage*" -LogicalSectorSizeDefault 4096 -PhysicalDisks (Get-PhysicalDisk).Where({$_.MediaType -eq "HDD" -and $_.CanPool -eq "True"})

New-StoragePool -FriendlyName HDD-Pool -StorageSubSystemFriendlyName "Windows Storage*" -LogicalSectorSizeDefault 4096 -PhysicalDisks (Get-PhysicalDisk).Where({$_.MediaType -eq "HDD" -and $_.CanPool -eq "True"})

EDIT:

I recommend testing New-VirtualDisk with "-WriteCacheSize", "-Interleave" and "-AllocationUnitSize" which gives best performance.

For example in your case:  "-WriteCacheSize 1GB -Interleave 8KB -AllocationUnitSize 512MB", and then format the news vdisk with 64 KB NTFS cluster size, that may give best performance. The Microsoft defaults for partiy are not well chosen.

If you want to use dual parity you may have to test more extensively since the NTFS cluster size cannot be divided by 7, or use 10 HDDs.