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Windows NT Server has a feature, called fault-tolerant disks, that is essentially software RAID on logical drives. The feature was dropped in later Microsoft operating systems because of the superiority (and rapidly falling price) of hardware RAID systems.
While Windows 2000 can recognize fault-tolerant disk partitions for compatibility reasons, the newer operating systems, such as Windows XP and Windows Server 2003, can't. That's why fault-tolerant disks can still bite you when you upgrade a Windows 2000 system to Windows XP or Windows 2003. If you install these operating systems on a computer that is still using a fault-tolerant logical drive, the new operating system won't be able to find the fault-tolerant disk.
The fix isn't complicated, but the problem can be infuriating. Obviously, if you realized there was a fault-tolerant disk on the computer, you would have done something about it before you installed the new operating system. Now you've got a perfectly good logical disk that isn't recognized by an operating system that otherwise works perfectly.
The best solution -- other than replacing some elderly and doubtless arthritic hard disks -- is to convert any fault-tolerant disks to dynamic disks or back up their data to be restored later before installing the new OS.
However, if you blithely installed the new operating system without doing that, and you need the data on those disks, you can recover it by using Microsoft's ftonline utility.
Strictly a recovery tool, ftonline makes the partition visible and allows you to back up the data on it. Once that is done, you should reboot the system, use Disk Manager to delete the fault-tolerant partition, recreate the disk either as a basic or dynamic disk and restore the data.
Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last 20 years he has been a freelance writer specializing in storage and other computer issues.