  Neither.  It is not currently a participating member of an array, but foreign does not mean it is "totally not working".  The controller detected a configuration on the drive that did not match the other(s) in the array, so it marks it as foreign until the admin can determine the best course of action for it.  This is absolutely a dangerous thing to do (leave it to the admin to know the correct thing to do with it), but this prevents the controller from having to make live/die decisions regarding the data, making it the admin's responsibility to learn the technology required to manage it. You have two options with foreign configurations: IMPORT - Should only ever be used in the event of multiple drives being marked foreign simultaneously, including moving drives to a new controller/system, etc. and should only be used when an array is offline. CLEAR - Should be used whenever the array is online and only a single disk is marked foreign (or multiple disks in the event it is a RAID 10/50/60 but the array is still online); this is usually the case when a single disk fails (or is failing) or simply goes offline as a result of a timeout, etc. and is often the case when replacing a failed disk in an array.  Only after clearing the foreign configuration can the drive be made a hot-spare to the array in order to rebuild the drive into the array.
Hello,  I have replaced a RAID-5 disk with a spare disk that already has a Foreign Config on it. I want to clear the config by hand, can i zero out the disk so it will be detected as empty? (dd if=/dev/null of=/dev/sda) Best regards,  Eddy
Perc 6/i Foreign config on disk / how to clear a disk