To bring a foreign disk back to an online state depends on the original context of the disk.
The following describe the various contexts:
Use Import Dynamic Disk Group to bring a Foreign disk online if the disk was part of a dynamic disk group that was created on another computer and moved to the current computer.
To add a foreign disk that was created on another computer
A dialog box comes up that displays the name of the dynamic disk group.
The disk group then imports. All existing volumes on the disk are visible and accessible.
More information on importing and deporting dynamic disk groups is available.
Use Import Dynamic Disk Group if the Foreign disk has a secondary dynamic disk group (that is, a dynamic disk group other than the dynamic disk group containing the computer's boot or system disk) and you have switched between operating systems on a dual-boot computer.
When you have one or more secondary dynamic disk groups on a disk in a dual-boot environment, the disk is marked as Foreign when you switch between operating systems, because the secondary disk groups are not automatically imported. A shared primary dynamic disk group on a disk in that situation is automatically imported.
Use Merge Foreign Disk if the disk was originally created on the current computer but was removed and now you have reattached it to the current computer and want to restore it to its status as a member its original dynamic disk group.