Hi,
If you are simply wanting to use an external hard drive(s) to store / backup files etc I have to ask why you are considering a RAID configurable Dual Hard-drive box ?
RAID can be viewed simply as storing data across both drives [ i.e. treating both drives as one ]
It does have one or two advantages - the main one being faster read / write capability.
However, if you are using it for back-up this will not be important to you.
For back-up purposes I use an external hard-drive(s) connected directly to a USB port on my computers.
No dual box needed.
USB3 is backwards compatible.
What this means is - yes you can use any device that is USB 3 on any computer with a USB Port.
If your computer has USB 2.0 then a USB 3.0 device will work BUT it will work slower - i.e it will run at the speed of your USB 2.0.
Molex connections are usually internal 12v connections from the Power Supply Unit on a Desktop Computer. [ a tower type - not a laptop ]
Are you considering a permanent internal arrangement ?
I can't see why an external Dual HDD Box would need a Molex connection.
1. Would an external Hard Drive serve your purpose.?
2. Do you need RAID ?
3. Can you provide a link to the device you are considering please ?