If you weren’t aware of it, the current generation of Airport Extreme (802.11ac version) officially supports Time Machine backups to drives connected to the USB port.
But which drive to get? Some drives work strangely with the USB port on the Time Capsule since they try to draw a lot of power. On the other hand, it’s sort of a nice idea to avoid drives with external power supplies. It can be really inconvenient to find extra space for a wall wart that has to wire all the way back to the drive next to your Airport Extreme.
Well, I took a chance on the Seagate Backup Plus Slim 4TB Portable External Hard Drive … and it works admirably. It runs off USB power alone, which makes it easy to simply place next to your Airport Extreme regardless of where the power outlet is. Just be sure to format the drive on your Mac before you hook it up to the Time Capsule, since it defaults to NTFS. Enjoy!