I think every Linux installer allows you to mount any existing partition for /home, /boot etc but Slackware is not modern.
It would have been a lot eaiser if Slackware were installed first as Mandriva installer is a lot newer and therefore more flexible.
You can let the Slackware installer to set up the dual boot for you but I tend to do it manually as it is a lot simpler.
Steps to dual boot Mandriva with Slackware
(1) While inside Mandriva, assuming it is installed in hda1, you duplicate its boot loader inside it root partition by a terminal command (needs root privilege so preecd command with sudo if you don't have)
Code:
grub-install /dev/hda1
I assume you are using Mandriva standard boot loader Grub. If it uses Lilo the equivalent terminal command
The above makes Mandriva chainloadable by Lilo from Slackware.
(2) Install Slackware, let its Lilo into the MBR.
(3) Add these line to Slackware's /etc/lilo.conf
Code:
other=/dev/hda1
label=Mandriva_in_hda1
(4) Run Lilo again by just
so that Lilo compiles the new setting.
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Alternatively you can let Mandriva stays in the MBR to dual boot SLackware too. The steps are
(a) Install SLackware but instruct the installer to put Lilo inside the root partition of slackware. I assume you install Slackware in hdc1 and this will be know as (hd1,0) to Grub for being the 1st partition of the 2nd disk, as Grub counts from 0.
(b) Add these lines to Mandriva's /boot/grub/menu.lst
Code:
title My Slackware in hdc1
root (hd1,0)
chainloader +1
Piece of cake, isn't it?
In fact you can set it up to jump from Mandriva to Slackaware and then back to Mandriva if you want, to boot any of them in either Grub or Lilo.
Booting is child play in Linux. You can quote me on this. I just hate to see the word "dual boot" because it is only for the inferior systems like MS WIndows. In Linux every boot loader was born to multi boot.