Odd disk spin-ups (seperate non-OS-disk)

Hello,

I have -current installed on a SSD (incl. /var) and have another, 
usually not used, hard disk which gets spun down after some minutes idle 
time.
Frequently my complete system stops (no mouse moving) until the not used 
hard disk spins up.
How can I find out what process causes the disk access and more 
important, why does it block my complete system?

Best regards,

-Harry