I have a SQL 2005 server with 4 LUNs attached via the Microsoft iSCSI initiator:
1. System databases
2. My databases (seven of them)
4. Snap which is required when using Snapmanager for SQL
In the next few weeks I will be bringing up a new SQL 2008 R2 server and slowly migrating all of my databases from one SQL server to the other. The question is can I use the same 4 LUNs for both servers or should I carve out 4 new LUNs for the new SQL box? I do know that I need to purchase one more license for Snapmanager for SQL to use on the new server. I use an FAS3020 for storage. All of my VMs (including my SQL server which runs as a VM) use NFS storage. I only use iSCSI luns for SQL and Exchange.
Migration of data is always tricky, yet as already said in this thread, Windows architecture doesn't allow accessing the same LUN from two different hosts at any given time (with the exception of Hyper-V, but that's completely different story).
Personally I'd detach databases from the old server, disconnect LUNs, mount them to the new one & simply upgrade databases from 2005 to 2008 (via SQL Management Studio).
Needless to say, doing backup upfront is strongly recommended! (as usual )