Upgrade Time Capsule with a 6TB WD Green HDD: This is a summary describing an attempt at upgrading the HDD in a Time Capsule A1470 from 2TB to 6TB - just in case it proves useful to someone else attempting the same task, or contemplating it. There may well be better approaches - but here goes: Entry: The base is held in place by twelve plastic retention lugs evenly spaced around the inside perimeter. I slide a thin strong spudger between the base and the body (not too far in) to gently prise the edge of the base up, while trying not to mark or dent the soft plastic. There are cables in "too close for comfort" proximity to the lugs at the front and back of the unit - so starting at the left or right side seems a safer bet. The white plastic casing flexes alarmingly, causing cowardly postponement until the following day… With one spudger in, I introduce a second into the gap to reduce 'single-point' stress and help things along. Once one side is unclipped, things get a touch easier - working gradually around...
The introduction says it's a 2TB Seagate drive. I don't know what the original drive is in the 3TB Time Capsule, but see philgrocks and chrislewis1's comments posted above for a couple of replacement suggestions. This Time Capsule design A1470 has been available for several years, so it is possible that the internal drive used may vary over this period. The one removed from mine was a Seagate Barracuda 2000GB, Model ST2000DM001.
"Launch Airport Utility to check the disk is recognised OK, and erase it if necessary." Try repeating that last step in Airport Utility - your TimeCapsule may need to erase the disk itself before use.