You can install Windows 7 on the separate drive and do not need to
disconnect the Ubuntu one. You might want to switch cables so that
Windows will appear first and be the traditional C: drive. Otherwise
it will want to format your C drive at regular intervals (which will
have Ubuntu on it). If Ubuntu is on D: then it seems to ignore it.
Some applications will only install to C: in Windows.

You will need the Ubuntu live disk to fix grub because it will have
Ubuntu configured to boot from sda when it is now sdb. It is an easy
thing to do. You will likely have to do this in any case. The rule of
thumb is install Windows first and Linux second. The only advantage of
disconnecting the Ubuntu drive as far as I can see is to simplify the
process, as you will not get confused with drive letters.

You can write grub to either disk or both. My preferred method with
two drives is to leave Windows with the Windows bootloader and write
grub to the second drive. You can only boot into Linux with this
method by pressing a key to get to the second drive by a boot manager.
It can be a nuisance if you regularly shut off the computer, so in
that case I would just write GRUB to sda or C: which is the
traditional location. I use a desktop computer which runs 24/7 and
seldom re-boots. I like it that way for security. A nosy person who
chose to power off from the locked screen would discover that it would
boot into Windows which has nothing on it. They could not get into
Windows either since it is password protected.

I regularly rewrite grub. I install dozens of distros and each one
rewrites GRUB to make itself first and not all are equal. Some distros
do not recognize all distributions and make a grub entry for it.
Ubuntu has one of the best for that. I say this only to point out that
rewriting grub is a simple matter and not to freak out if you cannot
get into Ubuntu after installing something else.