What do I need to know before swapping in a new drive?

Ideally you should have a backup, do you have a copy of the data on the nas somewhere else? If its a backup then presumably there is a primary copy of all the data somewhere else.
USB backup is probably the easiest, with an NV you'd only be able to use a 2tb usb drive so all the data would need to fit into about 1.86tb, of course you could split the backup over 2 usb disks.
As for replacing the disk, you should do this ASAP.
Hot swap it - remove disk 3 (yes they are numbered from left to right, 1-4.
Then insert the new disk.
Don't format the new disk, the nas will do it.
It is wise to check the new disk for errors using a pc before inserting it, using vendor tools - for samsung drives you will probably need seatools from seagate or a generic hard drive testing tool like hard disk sentinel, which costs money.
The disk you are using is not on the compatibility list, but given that you already have one and it works, you should be OK.