Safe Dual Boot by Hardware Switching

The usual way of implementing a dual-boot system is by software. During the boot sequence the user selects the required operating system from a list. This is fine until one or other of the installed operating systems requires upgrading to a later version. Can we be confident that the dual boot will still work after the upgrade? At the very least we would be advised to backup everything.

There is an alternative which avoids all such worries; instead of switching between operating systems in software, use a hardware switch. Imagine a simple toggle switch on the front of your machine: UP for Ubuntu, DOWN for Windows – select the OS then power up.

I had Windows XP installed on my ‘best’ computer and Ubuntu on a second machine. The ‘best’ driving dual monitors, one of which (along with keyboard and mouse) was switchable between computers. What I really wanted was to make the best machine dual boot to take advantage of its performance and dual monitors. Windows XP is installed on one of its three hard drives.

I had available an 80Gb IDE hard drive. By installing this and using a 2-pole changeover switch to switch the 5V and 12V supplies between this and the Windows hard drive, Ubuntu and its associated program files could be installed on the 80Gb drive. The necessary wiring is easily made from a Molex 4-pin Y splitter. A simple terminal block can be used to extend the wires to the switch.

The diagram shows the Molex Y-splitter modified to switch power between the hard drives.

The red and yellow wires to the HDD2 connector are cut close to the incoming supply (male) connector, the other red and yellow wires are cut at a convenient point and the ends connected to the terminal block as shown. The choice of switch is up to you; I fitted a rotary switch to a blank front panel – this has the advantage that it is unlikely to be operated by accident. IT IS IMPORTANT THAT THE SWITCH IS ONLY OPERATED WHEN POWER IS OFF, otherwise data corruption could occur with serious consequences. The worst possible choice would be a rocker switch which might be operated accidentally when reaching for the CD drive. Other possibilities are to fit the switch to the rear of the case, or behind a protective cover. The normal procedure to change operating systems is to shut down, wait until the power led goes out, switch over and then power up again.

My Windows drive is a SATA drive and required a PATA (IDE) to SATA power adapter cable. (Note that SATA power connectors often have an orange wire for 3.3V in addition to the red and yellow. I believe that most drives do not use this at present but future drives may do. This would require an extra pole in the switch.)

The system works perfectly, the two other hard drives (unswitched) are accessible to both operating systems and can be used for shared data whilst the switched drives are each protected from the ‘other’ operating system.

Note: It might be necessary to modify the boot sequence in your BIOS to ensure that both hard drives are listed there.

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