The disk-spread assertion raced volume growth and heartbeats. volume.grow
-count is a writable-target topup, not add-N, and swallows partial-failure
errors, so one grow could leave a node's data on a single disk; ec.encode
then piles all that node's shards there and ec.balance can't spread them.
Retry grow on under-spread nodes until the master topology shows every node
holding volumes on at least two physical disks, then encode.
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