With the GiOConfig application installed you can determine whether the buttons on the GiO are transmitting data. Double-click and open GiOconfig from the Mac's Applications folder. Under the Expression pedal curve setting you'll see a Value: and grey rectangular bar. When pressing a button on the GiO the rectangular bar should turn blue and a Value of 127 displayed. This would indicate that GiO is transmitting MIDI data to the Mac and can be checked with all the buttons of the GiO.
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.