whether a partition or a separate drive makes no difference. But for my 2 cents worth. I would depend on a 3rd party boot manager and save yourself a lot of issues in modifying the Boot records.

I use BootIt NG for my dual booting and have so for like 5 years without any issues what so ever. Drives can be swapped or removed and new ones added very simply.

Boot Manager, Partition Manager, and Drive Image Utility - BootIt Next Generation

This one has some good reviews behind it and is free but I have never tested it. It works on the same principle as BootIt NG though

Partition Boot Manager - Reviews and free Partition Boot Manager downloads at Download.com