So it appears my SQL Installation was mucked up. Running a repair on it produced the following output.

  Setup has detected a non canonical discretionary access control list (DACL) on directory MSAS10_50.MSSQLSERVER.

Which is not the folder i was restoring to, but I assume something temporary was needing to be written.

Resetting the ACL on the folder corrected the restore issue. For good measure I reinstalled SQL as I feel uncertain as to the stablility of the install.

After this process the original restore db command for the SQL2005 db is now working successfully.

So it appears my SQL Server installation was mucked up. Running a repair on it produced the following output.

Setup has detected a non canonical discretionary access control list (DACL) on directory MSAS10_50.MSSQLSERVER.

Which is not the folder I was restoring to, but I assume something temporary was needing to be written.

Resetting the ACL on the folder corrected the restore issue. For good measure I reinstalled SQL Server as I feel uncertain as to the stability of the install.

After this process the original restore database command for the SQL Server 2005 database is now working successfully.