http://ubuntuforums.org/showthread.php?t=706813

Be aware that when installing Ubuntu to a second drive, that it will still modify the primary partition of your first drive. The master boot record will be changed and a bootloader installed there. This will allow you to select the OS you want to boot from, from a menu.

If you only want to try the OS out and don't think you'll use it permanently, then remove your main HDD first and install Linux on the secondary only. Then replace your main HDD again once the setup is done and use the boot menu in your system BIOS to boot from either HDD.

 

Caravel is right in his first paragraph. Although, I have my Ubuntu installed in a second hard drive and my Windows XP Pro on the first. I went to Windows XP pro and opened Disk Management. I completely eliminated any partitions and left the second drive with only Free space. I then partitioned the 2nd drive for the 3 partitions required in Ubuntu. That being, /, /home and swap. I allocated 2 GB for the swap partition. and divided the rest between / and /home. I then installed Ubuntu and directed it's installation to the 2nd hard drive. You have to be careful you don't select your first hard drive which will be designated hda1 with ntfs format. My second hard drive is designated hdb5. I manually installed Ubuntu into the second drive to avoid any problems. You have to be sure you back up all of your important data on your first hard drive just in case. You will see the 3 allocated partitions in terms of GB's when you go to manually install Ubuntu. Follow the on-screen instructions to install. Remember always keep in mind the hard drive you are using for this installation. If you have any doubts, back away until you are positive or then follow Caravel's 2nd paragraph. Good luck.