When you load any Computer you always write the primary Files required to start the system running from a HDD to the Default Boot Drive.
This is always the First HDD in the Drive Chain so if you have two IDE Drives it will be the Primary Chanel's Master HDD which is the Hardware Default Boot Drive and if you have a IDE and a SATA HDD it will be the IDE which is the Hardware Default HDD and if you have 2 SATA Drives it will be the drive connected tot he Lowest SATA Connector on the M'Board which is the Hardware Default Drive.
So if this Hardware Default Drive is removed the system is unable to Boot because the very basic Files which tell the hardware where to look for thew initial Files no longer is present.
If you must try to boot this system with one of the drives removed/disconnected or whatever not connected you need to install each OS individually with only 1 HDD int hew Computer.
However if you just want a Dual Boot Option which allows you to boot one or the other OS all you need do is install the Second OS to a blank Drive/Partition after the first is installed and allow the Windows Boot Manager to configure the Dual Boot Screen which gives you the option to chose a OS to load.
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