I read that 5400 drives spin slower so produce less noise?
In general, yes, although I would expect that there will be exceptions. The design of the enclosure can also significantly affect noise levels. Hopefully one of the others who may be using an OWC enclosure similar to the one you are considering will comment further.
Also this drive is "Raid" capable and since i have no prior knowledge of how to set a Raid system up should i be using the "mirror" setting for this drive?
Yes, you would want to use the RAID 1 "mirror" configuration.
Keep in mind, though, that although a RAID mirror protects against the possibility of data loss due to drive failure, a small but non-zero chance remains that something can happen that would cause the data on both drives to be lost simultaneously. For example, the controller circuitry in the enclosure or the operating system in the Mac going berserk, undiscovered design anomalies in the controller circuitry or its programming, the power supply going into an overvoltage condition, electrical surges, virus infections, etc. For anything really important it is desirable to have a second backup, IMO, which can be updated whenever it is convenient to do so.
Regards,
-- Al
