Moving large database to another drive, how to measure performance before, and setup some type of replication for fast fail back?
You can use perfmon to measure your disk performance and contention.  I would measure the following counters for the appropriate drives (all under physical disk):


Avg. Disk sec/Transfer - Otherwise known as IOPs, this is the total number of actions (Read and Write) that occur on your disk.
Avg. Disk sec/Read - This is the average amount of time it takes an action to read from your drive.  For DB operations, it's recommended (generally) this be under 20ms.
Avg. Disk sec/Write - Same as for sec/Read, but for writes.  Again, should be (generally) under 20ms.


You want to monitor these values over a period of time to see how they trend.  If you capture them at regular intervals (default is every 15 seconds), put them into Excel and calculate your Min, Max, and Average over a reasonable period of time for your standard work period to establish your current baseline.

Next up, for testing the drive, SQLIO is a good tool.  I'd probably combine it with the perfmon counters to measure how the disk performs in comparison to your current setup.  This will give you a general idea of how the disk will perform.

For replication, you can replicate databases and individual objects, but not files.  So to accomplish your goal, you would need to basically create a full copy of the database to replicate to and handle the different naming/connection strings.  That being said, I believe that's WAY overthinking the issue.  If you do enough baselining and testing up front, you will know how it will perform before you even make the move.