External storage options

RAID 1 = the size of whichever drive is smaller.

A 2TB and a 3TB drive in RAID 1 = a 2TB RAID.

Plus...only very recently are drives larger than 4TB appearing. The Buffalo thing might not know what to do with them.

Right from Wiki:

"It is recognized that disks are an inherently unreliable component of computer systems. Mirroring is a technique to allow a system to automatically maintain multiple copies of data so that in the event of a disk hardware failure a system can continue to process or quickly recover data. Mirroring may be done locally where it is specifically to cater for disk unreliability, or it may be done remotely where it forms part of a more sophisticated disaster recovery scheme, or it may be done both locally and remotely, especially for high availability systems. Normally data is mirrored onto physically identical drives, though the process can be applied to logical drives where the underlying physical format is hidden from the mirroring process.

Typically, mirroring is provided in either hardware solutions such as disk arrays, or in software within the operating system (such as Linux mdadm and device mapper).[1][2] Additionally, file systems like Btrfs or ZFS provide integrated data mirroring.[3][4] There are additional benefits from Btrfs, which maintains both data and metadata integrity checksums, making itself capable of detecting bad copies of blocks, and using mirrored data to pull up data from correct blocks.[5]

There are several scenarios for what happens when a disk fails. In a hot swap system, in the event of a disk failure, the system itself typically diagnoses a disk failure and signals a failure. Sophisticated systems may automatically activate a hot standby disk and use the remaining active disk to copy live data onto this disk. Alternatively, a new disk is installed and the data is copied to it. In less sophisticated systems, the system is operated on the remaining disk until a spare disk can be installed.

The copying of data from one side of a mirror pair to another is called rebuilding or, less commonly, resilvering.[6]

Mirroring can be performed site to site either by rapid data links, for example fibre optic links, which over distances of 500 m or so can maintain adequate performance to support real-time mirroring. Longer distances or slower links maintain mirrors using an asynchronous copying system. For remote disaster recovery systems, this mirroring may not be done by integrated systems but simply by additional applications on master and slave machines."