My 1 TB Time Capsule drive is full with Time Machine backups. I'd rather spend $130 on a 3 TB drive than $500 on a new 3 TB Time Capsule.
Is an external drive attached to a Time Capsule supported for Time Machine backups? I have read that trying to use Time Machine with an Airport Extreme and an attached external drive is not supported and is in fact unreliable. Does that extend to Time Capsules with an external drive?
I realize that a Time Machine backup cannot be split across drives — I would move entire backups to the 3 TB drive.
I bought my 1st generation Time Capsule (MB277LL/A) in 2008. I am running Mountain Lion.
Best Answer
Update: Now Officially Supported
At the time this was written (May 2013), the answer below was correct. Apple introduced new hardware, the Airport ExtremeBase Station (802.11ac) (6th generation) in June of 2013, and backing up to an external drive connected to an AirPort Extreme Base Station (802.11ac) or later AirPort Time Capsule is officially supported. Unfortunately, Apple no longer makes the Time Capsule or AirPort extreme. Also note that while for a long time Apple only supported network backups using AFP (Apple Filing Protocol), they have dropped AFP in favor of current SMB (that is, SMB3) protocols. This support article seems to be getting updates as the support situation evolves.
So, in 2019, your options for wireless backups appear to be a NAS with SMB3 support and advertised Time Machine compatibility or a Mac shared as a Time Machine backup destination (see support article linked above).
Original Answer
It is not officially supported. "Time Machine can’t backup to an external drive that's connected to an AirPort Extreme, Time Capsule, or a drive formatted for Microsoft Windows (NTFS or FAT format). (Click on "Setting up Time Machine backups using an external drive" to reveal the information.)(Update August 2019: the original link now redirects you to current documentation. For historical information, see also Disks that can be used with Time Machine) This is usually because of issues with the network file system not guaranteeing that writes have completed on disk before giving a "success" status.