Thank you for this guide. I very successfully replaced the original 3TB drive (after it failed) with a 4TB Seagate Barracuda HD. I followed the instructions and was extremely careful. Everything went very smoothly and the unit immediately fired up and worked after I reassembled it and plugged it in.
I added a few tips up above with a couple of steps to suggest some easy tools for the trickier parts of the HD replacement.
I replaced the drive with a Seagate Barracuda 4TB. With this drive, there was no need to trim any of the rubber bumpers (top or bottom) at all.
Connectors 2&3 are definitely delicate. They do unclip towards the direction of the bottom/base of the Time Capsule. I was successful with unclipping them using an improvised tool. I had the top of a cat food tin (one of those with a pull tab and then the entire top of the tin comes off) and I cut a long straight piece of this somewhat soft metal with a pair of scissors. With this long , narrow but still soft metal piece, I was able to gently get behind each of connectors 2&3 and gently twisted my metal strip to pop the connectors off.
Worked like a charm.
I just did the hard drive swap. Here are a few tips about this step.
I used a plastic spudger, a box cutter knife and some strips of an old plastic card (old credit card). The sharp, pointy tip of the box cutter can help get you started to find a gap between the base and the white plastic. Then once you have a gap, you can twist the box cutter blade a bit to wedge in the spudger to create a bigger gap. Now there is space to insert a few strips of the plastic card around the gaps to keep the gaps open as you work around the rest of the base.
Definitely start from the two sides of the base (rather than the front or back), there are no wires there to damage.