I have two 640Gbs drivers mirrored (array) on W7.
I am planning to uninstall the second drive (mirrored) and store it, so I can have it in case the original fails.
My question is if in this case the mirrored drive will load properly without the original drive?
If these disks are in RAID 1, you cannot simply remove one disk and expect the array to work. RAID will not work with just one disk. Consider making a backup onto a separate disk (in an external enclosure) and then storing that in a separate location.
I use two 1 TB disks in a RAID 1 array for data files in my computer, and then make regular backups onto another 1 TB external hard disk.