Haines Brown <haines@???> wrote:

>> You probably want to set this partition to unused (or whatever it's
>> called, it's a looong time since I last did this) so that it doesn't
>> appear in the mount point table (eventually in fstab of the new
>> install). I think what you are telling it is that you want sda1 to
>> mounted as / IN THIS INSTALLATION and that then clashes with your new
>> / (on sdc) that you're trying to install to.
>
> In "typical usage:" option when configuring the root partition, there is
> no option that would disable it. In any case, I don't want to disable
> it, but to be able to boot any disk and use its grub menu to boot it or
> any other disk. I've always been able to do that.

I think you may be confused about what this section is doing.

You are telling the installer a) what to do with a disk & it's partitions (eg, should it format a partition), and b) where they should be mounted.

So for your partitions on sda, you tell it NOT to format them and not to mount them anywhere - I certainly recall those being options in the Debian installer, you may need to go into expert mode. Alternatively, tell the installer NOT to format them and set a different mount point (eg /jessie, /jessie/boot, ...)

When you set a mount point, it's telling the installer two things : where to mount the filesystem during the install, and what to put in the installed system's fstab. If you tell it to do nothing with the partitions/filesystems on sda, then they will simply be left alone - but watch put for grub install later on, you don't want to damage the grub that's already installed.

So don't format the partitions on sda, don't mount them anywhere, and you'll end up with a new install on sdc that just ignores the system already on sda - but as mentioned, be careful when it comes to grub install time.

Then (I assume through the system BIOS) you'll be able to boot using the old system & it's grub on sda, or the new system & it's grub on sdc.