How to make/set-up a JBOD hard drive

Hi, I'm in desperate need to know about my problem. So, I have a 1TB Iomega external hard disk bought about 1,5 year ago. The actual hard disk is 2x 500GB Seagate hard disk. In the product box it says that the hard disk is set up as a JBOD drive, that is why it is shown as 1 hard drive only in the computer. Now the circuit board of my 1TB hard drive is broken and it is not under warranty anymore. I cannot access my files anymore. As a solution I put both my 2x 500GB drive into my computer which already have another 500GB hard drive with an OS of Windows 7 in it. In other words, I made my external drive into an internal drive. In my computer, both drive now appears as 2 separate hard drive and whenever I want to open it, it says that the drive need to be formatted.

The question is, how can I set up my hard drive to become a JBOD again? Or in other words, how can I make a (J)ust (A) (B)unch (O)f (D)isk hard drive? I really have to save the 600GB data on those hard drive. Could anyone help me out with this?