No boot after removing 2nd hd

Clean installed win 8, 1 hd, no problems worked well and installed all programs.
Three days later i added a second hd to backup to. All went well.??

If i remove the second hd the system wont boot, just hangs at m/b boot logo, put the 2nd hd back and boots normally.

Suggestion?

Check in BIOS, you may have HD 2 as first boot.