much grieve to owners of IBM drives when used in conjunction with
tagged command queuing, and didn't serve any purpose at all (since
experiments have proven that it simply didn't work).
Instead, call scsi_start_unit() once in sd_attach(), so in case the
drive has been configured to `remote start', it will spin up there.
(If it has spun down later, it must have been because of administrator
action (scsi(8)) anyway.)
While i was at it, bump the timeout for scsi_start_unit() to 30
seconds. 10 seconds were way too few for most drives.
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