The description of dfs.datanode.disk.check.timeout in hdfs-default.xml is as follows:
Maximum allowed time for a disk check to complete during DataNode
startup. If the check does not complete within this time interval
then the disk is declared as failed. This setting supports
multiple time unit suffixes as described in dfs.heartbeat.interval.
If no suffix is specified then milliseconds is assumed.
I don't think the value of this config is used only during DataNode startup. I think it's used whenever checking volumes.
The description is misleading so we need to correct it.