Cannot Create New Volume on Disk with unallocated space

Question

I have a server running Windows 2016 Core. I am trying to create a new volume on the existing disk which has over 2TB unallocated. It is a Raid 10 disk, basic and currently has a system reserved volume and 1 simple volume. The option to extend the exiting volume is available in Disk Management, but New Volume is not. If I try create a new volume using powershell, the error is that there isn't sufficient space available, whether I specify a size smaller than what is left unallocated or use the maximum size available.

I would prefer to create new volume rather than extend to reduce the risk. Any ideas why this could be happening?

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Thanks, Steven