External RAID with one drive? Different drives?

I'm in the market for a new enclosure as my 4 year old Rosewill (Rosewill RX81-MP-SC- BLK 3.5" SATA to USB & 1394 a/b & eSATA External Enclosure) has been disconnecting randomly from my iMac. Planned on getting the OWC Mercury Elite Pro but then saw an option for the RAID version that holds two drives for $5 less.
Link: http://eshop.macsales.com/item/OWC/MEPQ936AL2/
The drive supports RAID 0/1/SPAN and, according to them, comes configured as RAID 0.

Would I be able to run this as a single HDD enclosure until I purchase a second one?
Could I use a different HDD as a second (I have a spare 640 gig sitting in a drawer somewhere) and use the SPAN option there.
If no to the first two, do I need to purchase the exact same drive? I currently have a HITACHI Deskstar 5K3000 HDS5C3020ALA632 (0F12117) 2TB SATA 6.0Gb/s 3.5" Internal Hard Drive that I purchased last year. It has been running fine and passed disk scans but newer reviews on NewEgg scare me about these drives dying at very high rates.

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More about external raid drive drives
  1. I don't think you can run raid with a single drive, especially raid 0. you can use different size drives and even different drives in a span array since it's not a true array. For other raid types, it's recommended that you use the same make/model drives in order to ensure performance, otherwise you may have a slower drive trying to keep up with a faster drive. if using drives of different sizes, your array will be limited to the size of the smaller drive. So, a 640gig drive and a 320 gig drive in raid 0 will be 640gig total in size since the array will only use 320gig of the 640 gig drive and all of the 320 gig drive.
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