Currently, it's possible to aggregate ONE and GiO (using the OS X utility Audio MIDI Setup), but the system may be less tolerant to sleep/wake cycles of the computer, session sample rate changes, and hot-plugging. It's best to minimize these activities when aggregating. Keep in mind that with Logic 9 and GarageBand it's possible to assign the input to one device and the output to another without the need to aggregate the devices. In Logic Pro, for example, choose Logic Pro > Preferences > Audio, and set Output Device to ONE and Input Device to GiO.
It’s an accepted “best practice” of most audio software providers that audio files should be recorded on a hard drive other than the Mac’s Startup drive (i.e. the drive on which the operating system is installed). You can probably get away with recording a few tracks to your computer’s Startup disk, but for the best performance of your GiO recording system, record onto a separate ATA/IDE, SATA, or FireWire drive whose spindle speed is at least 7200 RPM.
GiO offers a choice of two sample rates, 44.1kHz or 48kHz. So, what’s the best sample rate to record your project at? It’s a good idea to avoid unnecessary sample rate conversion stages, so the answer is determined by the sample rate of media on which you plan to distribute your recording. If the final distribution media is CD, record at 44.1kHz. If the media is video or TV, most often 48 kHz is the best choice. If you’re part of a larger production chain, and aren’t sure, ask whomever is responsible for assembling the final product - they’ll undoubtedly appreciate the forethought.