How to run disk testing with multi-hdds at the same time about stressapptest HOT 4 OPEN

You may use the arguments 
-f /mnt/sda1/file.1 -f /mnt/sda1/file.2 -f /mnt/sdb1/file.1 -f /mnt/sdb1/file.2
Specifying file locations mounted on different HDDs. This argument set will run 
the test on 
sda and sdb, assuming they are mounted at /mnt/sd*.

Dear Nick
  thank you for your answer, according your answer, mean that multi-hdds testing must in file-mode ?  
  I just test another way using "-d /dev/sda -d /dev/sdb ..." but this way just support only 7 or 8 HDDs no more , and sometimes it can works on 8 hdds or not .. could you help me to find root issue.
                             thanks again

Dear Nick
  thank you for your answer, according your answer, mean that
multi-hdds testing must in file-mode ?
  I just test another way using "-d /dev/sda -d /dev/sdb ..." but this
way just support only 7 or 8 HDDs no more , and sometimes it can works
on 8 hdds or not .. could you help me to find root issue.
                             thanks again

I typically test multi-hdd in file mode. However -d disk mode should work as 
well. 
Do you have a log of the output available showing the error?  I don't currently 
have a system with >7 drives.