OK Got an update on this one....
The clustered SQL Server isnt configured correctly. The customer (or another 3rd Party who installed SQL cluster) has not configured the disk dependencies correctly for the clustered instance of SQL. The log disk isnt a dependant resource of the SQL Server, and the actual (existing) LDF's are on the same disk as the MDF's. Not a very fault tolerant solution to say the least.
I was trying to install the EV logs to the SAN/SQL Log disk (as anyone in their right mind would). Instead I installed the EV database and logs to the database disk and the install continues as normal.
I have dumped this config, however and will now be pushing the customer to correct the mistake before I continue with a best practice approach.
Andy