This is not EX1-specific, but this forum is full of really experienced people and one of the most busy ones, so here I go:

I'm configuring a new external storage to hold source mxf clips ONLY (plus some stuff that Vegas will inevitably write to it when editing from it, like the audio peaks files). The temp and render folders and alike will be held elsewhere

Since the clips are usually multi-gigabyte in size, which cluster size would be optimal? I'd think a one considerably greater than the NTFS default. Any opinions on that?

http://smallvoid.com/article/cluster-hard-disk-partition.html

For video, I would try 16k or 32k and write some files to it to test if there is any performance improvement. You can also use benchmark tool to test the HDD performance.