How can you reverse the effects of an ALTER DISKGROUP ... DROP DISK command if it has
already completed?
A. Issue the ALTER DISKGROUP ... ADD DISK command.
B. Issue the ALTER DISKGROUP ... UNDROP DISKS command.
P ... DROP DISK CANCEL command.
C. Issue the ALTER DISKGROU
D. Retrieve the disk from the Recycle Bin after the operation completes.
If the DROP DISK operation has already completed,you must use ALTER DIKGROUP_ADD
DISK to add the disk back to the disk group.In any case,the disk group is continuously available
to all users and no data is lost.
原文：http://www.cnblogs.com/Babylon/p/7803586.html