I don’t like quoting whole slabs of another blogs posts, but this is worth spreading to a wider audience:

… If you only use a single drive attached to an eSATA port, you can quit reading, but if you are like me and want the best performance possible, you have probably invested in a 2.5” or 3.5” external portable cabinet that can take two or more drives.

These cabinets from makers like StarTech and AMS, have built in hardware RAID functionality that allow you to configure the dual drives in a RAID configuration without requiring a RAID card in your machine. Typically RAID 1 (mirroring) and RAID 0 (striping) are the two most common approaches in a dual drive case.

BUT, these external cases rely on a technology called Port Multiplication to see all the drives. You can go read the detailed description at your favorite web site, but basically port multiplication allows you to have multiple drives accessible through a single eSATA port. The catch is that the eSATA interface must support Port Multiplication.

This is where the issue is with integrated eSATA ports in laptops and desktop motherboards.  They do not all support port multiplication…actually very few do. …

Robert Larson on Beware of Integrated eSATA ports