Ubuntu will recognise a pre-existing Windows installation and chainload the Windows bootloader to the Linux GRUB bootloader in most cases, unlike the other way around, where Windows will invariably ignore non-windows bootloaders, and overwrite the existing boot record with their own. Windows 8 does not natively recognize file systems that Linux often uses, only Microsoft's own FATxx and NTFS file systems, and a new one called ReFS.
The Windows 8 bootloader menu is becoming more flexible, being able to natively boot into Windows Image (.WIM) files and Virtual Hard Disks (.vhd) as well as bootable volumes and optical media.
There are different versions of the GRUB bootloader, which work slightly differently. The GRUB menus are editable text files, but require accurate syntax, and like many things in Linux, they may require some obscure knowledge to know how to work them to your best advantage, but there are many knowledgeable people online willing to offer help.
The safest way to install a new OS is to disconnect any other hard drives during the installation of the new OS. Afterwards, the other hard drive can be reconnected.
If the bios allows the user to choose which drive to boot from, this can be selected at boot time, usually by selecting one of the Function keys like F12 to bring up a menu, or if not, a free 3rd party boot manager can be installed on the first hard disk, such as the PLOP boot manager
which will allow almost any storage medium to become bootable, including USB flash memory and disk images such as .iso files on disk, even if the BIOS does not natively support it, offering a menu to boot from each attached drive.
