Run computer on external HDD to RAID two internal HDD

Hey guys, I'm new to ten forums but I've been on seven forums for a while but upgraded.
Here is my issue
I want to mirror one of my internal HDDs to my external HDD. Then I want to run my computer off of the external HDD so I can RAID my only two internal HDDs. Then transfer everything from the external HDD back onto the two RAIDed internal HDDs.
I've already looked at the storage spaces in the control panel and I don't see any way to successfully mirror the two drives without erasing all the data on the internal drive.
I'm not concerned about space; I have two 1TB internals and one 1TB external.
Any help will be great, Thanks.

[EDIT]
I came up with an idea to do this. Create a system image through Backup and Restore onto the external, then restart and enter BIOS and change the boot settings so that the external HDD with the system image is used as the primary boot option. If that works it should allow me to run the OS off the external HDD. Next go into storage spaces in the control panel and RAID the two internals. Once that is done I should be able to go in reverse and create a system image from the external onto the two RAIDed internals, then get back into BIOS and swap the primary boot option back to the internal drives.
I'll post later on to let you guys know how it worked out.

Just for information when using external disks - Windows has an explicit block on running from an external USB drive (running Not Installing), there are methods purported to get around this restriction, but the best way is to use eSATA