Attempting to Upgrade Internal Storage Capacity

[Huehuebewm]

I would like to upgrade my PC's internal storage up to 1TB. I noticed that said PC has SATA cables that can connect to a hard drive but I am unsure which type of hard drive is compatible. There are labels and jargon regarding computer hardware that I hardly understand if at all.  These are my expansion slots: 

• One PCIe x16 socket
• One PCIe x1 socket
• One M.2 socket 1, key A

• One M.2 socket 3, key M for SSD (PCIe x2)

I hope the above information is helpful. Thanks to any and all who choose to attempt to assist me. 

[WAWood]

According to the specs I could find, your PC does not have an existing HDD; instead, it has a 256GB M.2 SSD attached to the motherboard.

Any 3.5" HDD is going to be compatible with your PC, but HDDs are a LOT slower than SSDs, although they also cost a lot less.

So what are you looking for -- a LOT more space but slower (HDD) or some more space but with faster access (SSD)?