I've used two external HDDs (both NTFS) without issues. Just make sure that your external drives have sufficient power. External drives that come with their own enclosure should provide sufficient power. On the other hand, you may need to check your drives' datasheets if you are using a third-party enclosure to ensure that your drive will be sufficiently powered. (USB 2 ports provide 500mA max; some 2.5" drives -- particularly "high performance" ones made for laptops might exceed this limit)