canadian69

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May 1, 2010
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They are limited by the US bus speed throughput. (6 Gbits) Even at that the drives are unlikely to perform that well, so if you have USB 3.0 ports you will likely see MAX speeds in the 4-5 Gbit range.

Furthermore, it's nearly impossibly to tell what type of hard drive some of these external enclosures are actually using (and the same external product may use several different model drives".

My advice is to by your external USB enclosure and the your drive separately. That way you know what you are getting. Also, I would stay away from Western Digital.
 
Concur, Go for a seperate enclosure and for the HDD: For 3 1/2 in dives go with a WD 1TB Black or the Samsung F3 1 TB sata II drive. For 2 1/2 In Laptop drives go with 7200 RPM UNDER 1 TB.

Real life performance on USB3 can be a mixed bag:
.. The IB (Z77 MB) are the only MBs that offer native support for USB 3, previous CPUs/MBs use a 3rd party controller. Make sure the controller has the Latest firmware, and driver.
.. Real life performance will also very widely based on the files read/written, ie Sequencial large file transfers may be good, But doing a large write small file transfer - may me subpar.