XPS 8960, really have a 4TB max storage capacity?

Maximum storage is the specs of capacity Dell has tested and validated.  It's also what Dell has available to sell with the system at configurations.  You can add higher capacity with the maximum capacity available in the market after the purchase.  

@johnschwei ,

Welcome to the Dell Forums.  The 8960 can handle two HDDs.  As @Chino de Oro has already told you, you can add much larger capacity drives than what Dell sells directly for the model at time of purchase.

https://www.dell.com/support/manuals/en-ca/xps-8960-desktop/xps-8960-service-manual/installing-the-35-inch-hard-drive?guid=guid-a888fb0d-34a0-4977-a6dc-af26177766fa&lang=en-us

I have an old 8930 SE with three HDD drive bays. In one HDD bay, I have the 2 TB Seagate HDD with the computer from Dell when I ordered it, in addition to the 1 TB NVMe SSD. Subsequently, I ordered and installed a WD Black 4 TB in my second drive bay. One HDD bay is empty.

The system works great.  I am also video editing and use the 4 TB for storing my weekly system backups as well.  I hope this helps.  Have a great day.

Regards,

Phil

Sorry for sticking my nose in, but I did read that your need is for backup, and you really should backup to a destination outside your computer, like an external hard drive or (even better) an NAS.

The XPS 8960 has M.2 slots for two NVME SSDs, so why consider using a slow HDD?

Your XPS 8960 supports one of the following storage configurations:

• Up to two M.2 2230 or M.2 2280 PCIe NVMe solid-state drives
• Up to two M.2 2230 or M.2 2280 PCIe NVMe solid-state drives + up to two 3.5-inch hard drives