Adding drives, also known as scaling up, adds storage capacity and can improve performance. If you have available slots, you can add drives to each server to expand your storage capacity without adding servers. You can add cache drives or capacity drives independently at any time.
To scale up, connect the drives and verify that Windows discovers them. They should appear in the output of the Get-PhysicalDisk cmdlet in PowerShell with their CanPoolproperty set to True. If they show as CanPool = False, you can see why by checking their CannotPoolReason property.
Get-PhysicalDisk | Select SerialNumber, CanPool, CannotPoolReason
Within a short time, eligible drives will automatically be claimed by Storage Spaces Direct, added to the storage pool, and volumes will automatically be redistributed evenly across all the drives. At this point, you're finished and ready to extend your volumes or create new ones.
If the drives don't appear, manually scan for hardware changes. This can be done using Device Manager, under the Action menu. If they contain old data or metadata, consider reformatting them. This can be done using Disk Management or with the Reset-PhysicalDisk cmdlet.
Automatic pooling depends on you having only one pool. If you've circumvented the standard configuration to create multiple pools, you will need to add new drives to your preferred pool yourself using:
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