Foi caçar na net após o dica do KovBal sobre discos dinâmicos - e deparei com este tópico .
Basicamente, o RAID que usa discos rígidos externos USB não parece muito viável.
Uma citação aqui.
Using USB as the transfer technology you're only going to achieve a 30-Megabyte or less transfer rate. You seem to be going to a lot of effort for a marginal result. For external use, eSATA is a much better choice for performance.
Maybe I don't understand the intent, but having 3 USB external drives working as a single "Dynamic" volume is almost certainly going to saturate the Hubs/Controller they are on.
E outra citação aqui.
The overhead of software RAID will probably cause problems on USB drives. Even if you get it working I think it will probably be flaky and error prone. You can buy USB enclosures that you can install multiple drives in. These enclosures use an embedded Linux so that you can then setup a RAID array. This way the RAID calculations and read/writes are all done outside of Windows. The interface can be USB, SCSI, eSATA, LAN, whatever, it makes no difference. Software RAID is a compromise at best. I don't believe RAID 5 is supported in Vista in any case. I would look for a different way to achieve your goal.