My Apple Time Capsule 1TB MC343Z/A 3rd Gen Model A1355 Firmware 7.6.4 has gotten too small for all the backups and files, so it was time for an upgrade. As Time Capsules are still terribly expensive in the larger versions, I went for the DIY upgrade. Judging from some forum posts, the Western Digital RED line for NAS is compatible and suitable. In the end I settled for the WD40EFRX. I went with this rough guide although there is a much more detailed iFixit guide available. The most complicated part is getting the rubber from the base (I ripped mine a little so take your time), the rest is just trivially done in 10 minutes. After everything is done, AirPort Utility will alert you with a warning that a drive needs a repair. Under drives select the partition and select delete. It will now reformat your drive and you are good to go.