Why can't I run 2 x 2.5" drives at the same time?

[sacentre]

I hope someone can help me with this problem

My system uses

a 256GB M.2 SSD on the mobo for the OS
an 8TB 5.25" SATA drive for data on SATA Port 0
a DVD ROM drive on SATA Port 1

I want to add 2 x 4TB 2.5" SATA HDDs in a 5.25" caddy. They are connected to SATA Ports 2 and 3.  The drives were originally Seagate Backup Plus drives which I removed from their enclosures.

The problem is I can only run one of the 4TB drives at a time. If I try to run both either one or the other will hang when I try to copy to/from. Sometimes my display will go crazy and I get a green screen or the PC will hang altogether.

I've tried them in different SATA ports and even tried adding a 4 x Port SATA PCIe card to connect them to but the result is the same: one or the other runs fine but not two.  I've have no OC settings, just defaults.

The motherboard I'm using has 6 SATA3 ports so I assume it should be able to support this many drives.

Can anyone suggest what the problem could be?

Z170X-SOC FORCE motherboard
Intel i7 6700K CPU
32GB Vengeance DDR4 RAM
Cooler Master V850 PSU
Nvidia K4200 Graphics card

TIA
Trevor

[teknology9]

Hi,
 
You might find the link below useful:


https://forums.tomshardware.com/threads/does-my-motherboard-support-4-tb-hdd.3284108/

teknology9

[sacentre]

Thanks, but my problem has nothing to do with capacities. All the drives are GPT. The issue is that problems start when I try to run the two drives together. If I switch one off, there's not problem. It's a if there's insufficient power for two at once. Later I'll try connecting both drives directly to the mobo with individual cables instead of in the drive caddy to see if this makes any difference.