After u plugged in ur storage drive, go into the BIOS and check whether u can detect both HDD and if so, check the boot sequence whether it is correct.
Let's say u have SATA_0 and SATA_1, it is always advisable to connect ur boot drive to SATA_0 and storage to SATA_1. If for some reason that you have to connect the other way around due to certain reason ( like i did in my rig ), make sure your OS HDD is on the list of boot sequence and it is in the correct order.
If everything else is in order as above, the another possibility is that ur storage drive drive might have died. The only way to find out abt ur storage HDD is through the external casing or sata to usb cable like this one http://www.newegg.com/Product/Product.aspx?Item=N82E16812232002.