Toki said. Though most of us recomended using two HD, but SONAR it self will work just fine with single HD even 5400 RPM. I've got two HD one 5400 and one 7200 in my system. But once I tried use only 1 HD, it doesn't make the SONAR stop. Just consider the data integrity, writing accuracy, and more stable performance on system with 2 HD, specially if you work with a bunch of data (large number of tracks / long songs). Hey... where's moskus ?