[ubuntu] Dual boot on twin drive machine

I'm trying to install Ubuntu 12.04 as a dual-boot with 10.10 (64-bit in case it's relevant).

I've got two hard drives, a 750GB with the operating system on it (not partitioned as yet) and a partitioned 1TB drive containing most of my files.

The trouble is that, though it recognises the existing 10.10 installation, setup insists on trying to install 12.04 to the larger drive. The smaller drive isn't even shown as being available unless I click on "advanced setup", where I'm completely lost. I don't understand most of what I see, and most of the options shown don't seem to be available anyway.

I want to try a full installation of 12.04 while keeping 10.10 in reserve in case anything goes wrong or I change my mind, but I want both installations on the same drive.

I don't want to physically disconnect the larger drive, and I'm not comfortable making changes in the BIOS, assuming I could disable (and then re-enable) the drive from there.

Any suggestions anyone? In case it's not obvious, I don't really know what I'm doing, so any help will be gratefully received.