XFileSharing Pro - Mod nginx has a strange problem

[xplan]

on my file server has 3 harddisk

1.hd1 and hd2 when download with accelarator ( IDM ) can split only 2 parts. while hd3 split to 8 parts

2.cannot download in parallel with hd1 and hd2 but
-hd1 and hd3 can download in parallel
-hd2 and hd3 can download in parallel

please help