source
http://www.hitachigst.com/tech/techlib.nsf/techdocs/97C736A5F94D66338625713A007D979C/$file/T7K500_DS.pdf

Both T7K250 & T7K500 250 2 discs 4 heads according to this & my results.

The full picture

T7K250 HDTach result from google search


My T7K500 with SATA1 connection