Mark Mirror Disk as Synchronized
To provide fault tolerance for RAID 1 (Mirror Array) and RAID 0+1, useful data on all Mirror disks must be same as their corresponding Source disks. But we don't care whether un-useful data on the source and mirror disks are identical or not. For example, we use two disks to create a new Mirror array, both disks already have some data before they join the new Mirror array. Although data on these two disks maybe obviously different, we can still say this source and mirror disk pair is synchronized except that you want to keep and use the old data on the source disk. If the old data is not kept, then there will be no useful data on this pair of disks and thus they are synchronized.
The Mark Mirror Disk as Synchronized function
does give us an opportunity to tell the RAID application that we know a
pair of source and mirror disks are synchronized despite the fact that the
application may have found some different data blocks on them. Application will use a 
icon to represent an un-sync mirror disk (note: data on the mirror disk maybe
older or newer). If you are sure that all useful data on
this disk pair are identical, you can right click the
mirror disk's icon in Array View, and execute the Mark Mirror
Disk as Synchronized command on the shortcut menu, as shown in
below (an example of RAID 1, the operation is same for RAID 0+1 ) :
After the operation, the
mirror disk's icon will change back to the normal one: 
.