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Grub (the bootloader Ubuntu and most Linux distributions use) is a lot more capable at detecting (and loading) Windows than Windows' bootloader is at detecting Linux. I would personally overwrite the Windows bootloader with Grub.

Otherwise the safest way would be to stick a new disk in there, unplug the Windows dive and install Ubuntu on the new disk. After that's done, plug the old drive back in and use BIOS to determine which one boots. That would guarantee the Windows partitions aren't touched at all during installation.

Ubuntu's hardware support is pretty awesome (infinitely better than Win2k's out-the-box hardware support) but I can't guarantee your tape drive will work as you haven't specified its make or model. Put them in google prefixed with "Ubuntu" and you'll find out if people have been having problems.

Two things, firstly for >2TB you need to use a GPT partition type (right click on the drive number) but secondly BIOS-based systems can't boot from GPT partitions, only EFI-based machines - your ML is BIOS based. Luckily you have a P410i inside that box, you should be able to use the use ACU/ORCA to carve up your R10 array into two logicial drives, a small one for boot and the rest for your data drive, this latter one can use GPT just fine.