Hi Ian,
You may not be aware, but
WIN SERVER 2008 any version will not like being run from a non C: boot drive!! That violates just about every rule in the book for Server 2008 installation in Microsoft documentation as well as SDK kit policy. So unless you are a developer or just foolin' around (I have a Server 2008 running on a dell poweredge 2650 server; sort of, PSU died last year), what you are trying to do is a no-go.
Server's are
NOT designed for dual-boot with a standard retail home user version of Windows. Period. Especially, if this server or pc you are attempting this is located in a business or enterprise location. Not a Best-Practice move there friend.
If you are a developer or a foolin' around person here; you can get the Win7 dual boot to work (sort of!) by installing the Server 2008 on the C: Drive and the Win7 on separate partition also on the C: drive as long as you install the Win7
AFTER the Server2008.
Server 2008 disk management won't take kindly to running another OS, whether Windows or Linux or Unix, or Solaris or whatever on an external drive. In any case, you are attempting to run an unsupported configuration, so it may or may not work. Certainly, if you've gone through certification classes on server, you would know this. If not, it's a new thing to learn.
<<<BIGBEARJEDI>>>