Internal HDD 6TB disappears when computer wake up from sleep mode

Sir/Madam,

I have dell optiplex 9010  MT with win7 -x64 ultimate installed . I mounted 2 nos  Hard disk for the storage purpose . Seagate 4TB and 6TB . Both are GPT partitioned .  I noticed 6 TB HDD is disappeared when computer wakes up from sleep mode. I need to restart computer in order to display it .

I  reset the BIOS  2) updated the BIOS to latest one 3) Installed seagate HDD driver for 6 TB 4) Ran "check disk " by going to that particular disk drive properties..  Nothing solved my problem.  I am sure it is not driver issue because nothing shows up in exclamation mark  in device manager . I ran the dell hardware diagnostics .All drivers are upto date . Only 6 TB HDD disappears only when it resumes from sleep mode. Both 4TB and 6TB are brand new HDD and i have already transferred 1 TB files .  Your help would be highly appreciated.