Premiere Playback Issues

Hey guys,

Just wanted to pick your brains to see if any of you can figure out why I'm getting such bad performance from PP (ver. 12lately.

Recently started a feature and on occasion my machine is struggling to playback even low res proxies.

Tech Specs below...

Mac Pro (Late 2013)

Processor   3.5 Ghz 6-Core Intel Xeon E5

Memory    64gb 1866 MHz DDR3

Graphics    Dual AMD FirePro D700 6144 MB

I'm working with Alexa 3k footage in a match sequence timeline off 2 8TB G-Raids @7200 RPM over USB 3 connections.

So my machine clearly should be able to handle full res playback of even the master files but as I said above sometimes I struggle to playback low res proxies at half or even quarter debayer.

Even though I'm not connected via Thunderbolt, or working off an SSD, I figured the fast G-Raids wouldn't be a problem but the light on whichever drive is being referenced is blinking when I start to drop frames.

They are also brand new production drives so I don't think they're the problem.

Thanks in advance....