Installing 2nd hard drive in a Dell Inspiron 580 MT

Hi,

Our 580 MT is 6 years old so time to instal a new Hard Drive before the original shows any signs of failing.

I have a Seagate 1 TB SSHD to become the main drive (& keep the original - now 6 year old - HDD in place as a Data Drive), 

My plan is to put the new drive in the second Bay, clone the Drive, & then simply swap the SATA cables so the "new" drive becomes the Main drive without having to alter anything in the BIOS

Of course this requires a third SATA cable attached to the Motherboard

I checked inside the Tower today, & as should be, two of the SATA sockets are in use, (Numbered 1 & 2)

Sorry for what might be a Dumb Question, but although the sockets marked 1, 2, & 4 are black in colour, number 3 is white - is there any significance to this?

Thanks in advance for any answers!