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greenskinmonster mynas> mdadm --manage /dev/md2 --re-add /dev/sde3
mdadm: --re-add for /dev/sde3 to /dev/md2 is not possible
如果 mdadm --manage --help ，没有看到 re-add 这个 option ，但在我的 ubuntu 18.04 是有的，不知道是否是因为这个 NAS 系统比较老的原因，我还在用 5.2 ，因为一直用着很稳定，加上黑裙，所以不想升级。
mynas> mdadm --manage --help
Usage: mdadm arraydevice options component devices...
This usage is for managing the component devices within an array.
The --manage option is not needed and is assumed if the first argument
is a device name or a management option.
The first device listed will be taken to be an md array device, and
subsequent devices are (potential) components of that array.
Options that are valid with management mode are:
--add -a : hotadd subsequent devices to the array
--remove -r : remove subsequent devices, which must not be active
--fail -f : mark subsequent devices a faulty
--set-faulty : same as --fail
--run -R : start a partially built array
--stop -S : deactivate array, releasing all resources
--readonly -o : mark array as readonly
--readwrite -w : mark array as readwrite