Autoreseed, now what?

Yes, you'll need to configure your spare using the same configuration as the failed drive. 

So, bring the new disk online (e.g., 

Get-Disk | Where IsOffline -eq $true | Set-Disk -IsOffline $false

Then, delete the empty mount point.

Then, create a new simple volume formatted as ReFS with a 64k allocation unit, and label the drive using the failed drive's label.

Next, verify the disk is recognized as a spare. Start with the server AutoDAG properties (e.g.,

Get-MailboxServer <Server> | FL *Auto*

Examine the output for accuracy, especially AutoDagVolumesRootFolderPath, AutoDagDatabaseCopiesPerVolume, and AutoDagTotalNumberOfDatabases.

Confirm that Exchange sees the new volume.

Get-Volume | Where FileSystem -eq ReFS

Make sure the mount point is under the AutoDAG root (e.g., C:\ExchangeVolumes\).

Then, resume the copies that were affected.  This will cause them to be in the FailedandSuspended state and then AutoReseed should do its thing.

If that doesn't work, then manually reseed the copies:

Update-MailboxDatabaseCopy <DB\Server> -DeleteExistingFiles

Hope this helps!