Proceed with caution. What I'm outlining here requires some expertise in handling electronic components including hard drives, and also using a wire stripper and soldering gun and having access to several feet of appropriate wire and a double pole double throw ( a DPDT) toggle switch - without the center off position - that kind is usable but inconvenient.

If you have any doubts about your ability to make this switching system, don't try it. I've worked with electronics and wires for so many years that it's second nature to me - and yet I am not immune to mistakes. I'm prepared to deal with mistakes. I have backups of everything, both hardware and software. 
In addition to having extra hardware, I regularly clone my hard drives (I use Norton Ghost 2003 - it clones my dual boot Windows and Ubuntu drive with no problems). If I ruin something when experimenting, I still have an up to date copy of all my files plus the extra hardware. If I destroy something, I'm not hindered. You may not be prepared for the consequences of making mistakes so proceed with caution.

(not for laptop computers).

Purpose of this switch. For reasons of your own, you may want two separate operating systems on two different hard drives, independent of each other, on one computer. One reason could be that there is one computer and two or more family members; one family member wants to use Linux and the other(s) wants to use another operating system. You want Linux on one drive and the other operating system on the other drive. You don't want both these hard drives in use at the same time, such as in dual booting. You want the two drives completely independent of each other - each drive must be booted independent of the other, which requires a  complete shutdown, flip the switch, then boot up.

One solution is to open your computer box each time you want to switch drives and manually remove the cables from one drive and connect them to the other drive. That's not very convenient. I found a better way that works with my combination of motherboards and hard drives: Asus A7S266-VM/U2, Biostar M7VKQ, PC Chips M810LMR, and a combination of Western Digital and Maxtor ATA hard drives.

Here's where I started. I had one hard drive with Windows 98SE installed and another hard drive with Ubuntu 6.06LTS installed. Both operating systems had been installed independently on a single drive, set as Cable Select and connected to the end position of the data cable, with nothing connected to the second position of the data cable. I wondered if I could connect both these drives permanently to the data cable and boot either drive separately by removing the power cable from the other drive.

To test whether it would work, I tried different combinations. With Win98SE on the end connector of the data cable and Ubuntu on the same data cable, second connector and the power cable connected to only the Ubuntu drive, Ubuntu wouldn't boot when in the second position of the data cable.

(UPDATE: Ubuntu does boot on either the first or the second data cable connector. I must have left the drive jumper on CS (Cable Select) in earlier test and the BIOS saw it as a Slave; today I placed the second drive jumper on Master and Ubuntu boots on either data cable connector.)
So either drive boots independent of the other, depending on which drive has the power cable connected. I haven't tried this method with any operating system other than Windows 98SE and Ubuntu Linux.

So with the Ubuntu drive connected to one position of the data cable and Win98SE connected to the other position of the data cable, either drive is bootable by simply having the desired drive connected to power while leaving the other drive disconnected from power.  

Just remember, you will have to set the second hard drive jumper to master; NOT slave and NOT CS.

With the connections as outlined above, I could boot the two hard drives as desired by simply leaving the data cables connected while adding or removing power to the drives by plugging and unplugging the power connector. But there is an easier way to do the switching by using a DPDT (double pole double throw) toggle switch to select which drive receives power. On the next pages I'll show you how I made my switch.
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This Web site is authored and maintained by Howard D. Dickens (Doug) and constructed using Web Studio. Copyright © 2007 - 2009.

January 17, 2007
Before touching any electronic circuit boards, read the warning on Page 2.

A gentleman named Rustam built this switch. He reported that he had very little experience but was able to get it done - and correctly. He mounted the toggle switch on the front of his computer for easy access.

Good job, Rustam and thank you for the picture.
Doug, March 2009.