Disk Mirroring (RAID Level 1) in Windows NT.

In disk mirroring, partitions on two drives store identical information so that
one is the mirror of the other. All data written to the partition on the primary
disk is also written to the mirror, or secondary, partition. If one disk fails,
the system is able to use the data from the other disk. Fault tolerance is a
reason to use raid 1. If the data on the mirror is primarily used for read
access, there is a speed advantage to the mirror single there are two
independent copies on two hard drives where a sector can be read. If the data
has heavy write characteristics, mirroring will place a performance hit since
the data must be written twice. For disk mirroring under Windows NT:

NT workstation does not support fault tolerance. Only Windows NT Server can
create and break mirror sets.
Mirrors are file system independent. Any partition with an Windows NT file
system (FAT or NTFS) can be used to create a mirror.
Mirrors are not dependent on disk geometry. The only requirement is that
free disk space used to place the mirror on be equal to or greater than the size
of the primary partition. Mirroring is not restricted to a partition of
identical geometry (size, number of heads, cylinders, tracks, sectors, etc.) nor
is it restricted to a drive of the same type (IDE, ESDI, SCSI, etc.).
Primary and mirror partitions must be on separate hard disk drives. They
cannot be on the same physical hard disk drive.
A single mirror set is limited to two hard disks only. Use disk striping
with parity if fault tolerance over more than two disks is needed.
When a mirror set is created, both partitions are assigned the same drive
letter.
Mirroring is the only Windows NT fault tolerant option available for use on
boot and system partitions.
If the boot or system partition is mirrored and the primary partition is
damaged, the computer can boot off the secondary or mirror partition by using a
fault tolerant boot floppy disk.
Only the Windows NT Server installation that created the mirror set will
normally recognize it. Other operating systems will not recognize the mirrored
partition. MS-DOS will identify the partitions of the mirror as “Non-DOS”
partitions. Windows NT and other installations of Windows NT Server will
identify the primary and mirror partitions as having an “Unknown” file system
type in Disk Administrator.
A new installation of Windows NT cannot be installed on an existing mirror
set. During setup, when selecting the partition to install Windows NT on, setup
identifies the mirror set as “Windows NT Fault Tolerance.” If you attempt to
select this partition for installation, a message appears, stating that Windows
NT does not recognize this partition, and it must be deleted for setup to use
it.
The fault tolerance driver makes the loss of one partition in a mirror set
invisible; you will be able to read from and write to the remaining partition as
if the mirror set was healthy.
A key to determining the condition of a mirror set is the status bar in Disk
Administrator. When you select one of the partitions of a mirror set, Disk
Administrator displays information about the mirror in the lower left corner of
the window. The status indicators are:
- [HEALTHY] the mirror is in working order
- [NEW] appears immediately after the mirror set has been created in Disk
  Administrator, but before shutting down the system and actual generation of the
  mirror begins.
- [REGENERATING] is displayed when generation of the mirror set by the system
  has been started but is not yet complete.
- [RECOVERABLE] appears when either one of the partitions in the set has been
  lost but the other partition is undamaged. This message also appears when one
  partition loses synchronization with the other.
Disk mirroring provides better overall write performance than striping with
parity and better read performance in the event of a drive failure.