Yes well you could increase the size of your 2 M.2 SSD I'm guessing you have two 256gb SSD?
Most likely you could use one for your OS 256GB is more than enough for OS and a few programs that would leave you with an empty m.2 slot I think there only up to 1 tb with the m.2 SSD's
your single SATA 111 slot I think they are up to 2 TB hard drive that will still fit there.
They do make conversion adapters to replace the Optical Drive I don't have the details of which one you need but most people get them on ebay, I'm also not sure whether the optical post is SATA 1.5 I think but could be SATA 11 either one is more than fast enough for data storage. so you could get another 2TB there.
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I haven't tried anything TB yet. I have several USB 3.0 external 2.5 format hard drives with quick hard drives in them they'll get 90 to 120 mbs speeds nearly as fast a the same hard drive will do internally. I've had pretty good luck with WD Passport 2 TB USB 3.0 external drives.
G752VY-DH72 Win 10 Pro
512 GB M.2 Samsung 960 Pro
1 TB Samsung 850 pro 2.5 format
980m GTX 4 GB
32GB DDR 4 Standard RAM
Z97 PRO WiFi I7 4790K
Windows 10 Pro
Z97 -A
Windows 10 Pro