Comment 6 for bug 1307614

Roderick,
  To quote the page you referenced:

| Ubuntu Server (CLI) Installation
| 300 MHz x86 processor
| 192 MiB of system memory (RAM)
| 1 GB of disk space
| Graphics card and monitor capable of 640x480
| CD drive

1GB for server. so 2GB is *twice* the *recommended* (not required) amount.

The point is that determining which disk is "right" is more about how the system will be used than anything else. Curtin allows the user via config to specify the drives (by device name) that are considered for installation. Admittedly, its not very dynamic, and requires knowledge of the how the system you're installing on, but simply:
partitioning_commands:
  builtin: curtin block-meta --device=/dev/sdb simple

That overrides the 'builtin' partitioning command (which is 'block-meta simple'). You can accomplish that through maas vi editing the configs that it sends to curtin.

My argument boils down to this:
 The user is who knows what "the right" configuration is. Curtin did automatically choose a valid configuration. Coding a long list of hueristics to determine what the user wants is not ever going to satisfy all use case.