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A little bit confusing: RAID1 (mirror set) consists of two physical disks, but operating system sees it as only one disk. A proper device driver is responsible for writing the same data to both physical disks, so if one of them fails, all data are still accessible on second disk.
If you have one partition created on RAID array, then you will see it as one and only one drive (for example E:). Under common circumstances, you cannot access physical disks in RAID1 array separately. This is necessary to avoid data inconsistency; both physical disks have to contain the same data.
So it is strange to have two drive letters D: and E: for the same partition on the mirror set. It can be done using a registry hack; however it should not be necessary. You can try to redirect pagefile to C: drive, restart, then change drive letter of your mirrored partition to E: and test your applications. Changing drive letter has no effect on data, the partition itself and its content remains intact.
Furthermore, usually there is some application (Marvell, Intel, SiliconImage etc) for monitoring health of RAID arrays; if you have one, you may easily discover if array is inconsistent or if it contains failed physical disk. Then you may rebuild the array or replace the failed physical disk and then rebuild RAID1 array without loss of data.
Last edited by valihrach; 17 Dec 2015 at 10:39.