you may want to look at this section of the Aperture 3 User Manual: Working with Referenced Images
As TD said, "File -> Relocate Masters" will do the trick. Just some more hints:
How do I export my older raw files leaving just previews in my MBA while still keeping/working with my new files on my MBA?
Select all images of your older projects and relocate them to an external drive using the above command. Make sure the drive is formatted Mac OS X extended.
You will be able to browse and tag your images, even when the drive with the original image files is offline using the previews. I'd set the Preview preferences in a way, that your previews at least have the pixel size of the display of your MBA.
What hardware do I need? Also is it possible to share a master? For example my MBA and iMac perhaps?
That depends on the ports your MBA has - any directly plugged in drive will do, USB, Thunderbolt, Firewire. Compare the transmission speed, available disk space, and the weight of the drive. With your MBA you will want a size that you can easily carry.
Sharing the original files? It is is possible, but be careful. Original image files are best be left alone. If you share the original files between two libraries on different macs, don't delete images and originals from both libraries or use commands like "apply to original file".
I found Relocate Original (I guess its the same as Master),
It is : Original image Files have originally been called "Masters" in Aperture, but since Aperture 3.0 they are called "Originals". If a question does not state the exact Aperture version number, we now never know how to refer to the menu entries.
so every time I wand to add to it I use Consolidate Original?
"Relocate Originals" will move the original image files of the selected image versions from their current location to a new, different location. Use it, whenever you want to move originals to somewhere outside the Aperture library.
"Consolidate Original" will bring the referenced original back into the Aperure library and make it managed again. You consolidate the library, if you want to move the library to a new location or to back it up with all originals included.
Is using Time Capsule a bad idea? since you always need to resync big files??
A bad idea for what? Are you asking, if you can backup your Aperture Library with Time Machine? Or if you can store your originals on a Time Capsule?
You can backup your Aperture library with Time Machine; but I would quit Aperture before backing up; that makes it easier for TimeMachine to write incrementally only the changes to the library to the backup.
If you want to store your originals on the TimeCapsule, I'd connect a separate drive for them to the TimeCapsule. TimeMachine works better, if it has the whole drive for backup. I do not know, if the TimeCapsule network is fast enough for the originals. I do not have a TimeCapsule, but Network access is usually slower than working from a directly connected drive.
There are a number of things to consider
1. Your internal drive will eventually run out of space if you are continously adding pictures to A3. How fast depends on how your work flow works and how many pictures you are taking.
a. The fastest use of space is to store your masters in your library on your internal drive
b. A slower consumption of your internal drive space can be achieved with reference masters (put them on an external portable drive) with your library stored on your internal drive with large previews
c. An even slower consumption of your internal drive space caan be achieved by reducing the size of your previews
But if you take enough pictures, even if your masters are referenced, you will run out of space. My current A3 library for just 2012 is 150GB and NO masters are stored in it.
The only solution is to develop a strategy that keeps all of the masters off your internal drive and structure your libraries so that periodically you can move the whole library to an external drive for long term storage.
My solution has worked for years, but may not fit your needs. I tend to work on projects and when complete, put them into my archives. All of my archives are storaged on external drives. I can plug those drives into my computer when I need to see those projects when necessary.
1. I use the USB 2 or 3 Western Digital passport drives for portable drives. I have about 25 of them from 250 GB to 2TB in size which I use as my primary working source of my pictures. Each project or set of projects is treated as a separate library and yes, in this case all the pictures are managed by the library on the external drive. Nightly, I back up this external drive with Superduper. With this strategy, I can plug my portable drive into my laptop, my desktop, my wife's desktop or at any location that has a Mac with a current version of Aperture and be able to work on my pictures. I can also give my son a copy of the libraries relevant to him and his wife. If needed, I can have 2-4 of these little drives with me at all times (that is up to 8TB of picture data available to me wherever I am).
2. Once I am done with a project, I import the project to my Primary library that sits on the internal drive (2 TB drive) in my desktop and then immediately relocate the masters to an external 6TB thunderbolt drive. This allows me to keep my Primary library as small as possible, but it eventually becomes unwieldy. My Aperture libraries for 2001-2005, 2006, 2007, 2008, 2009, 2010 (all as referenced libraries) are stored on ab external drive while the masters are stored on separate external drives. These archived pictures and libraries have a backup and an archive backup. My internal drive on my desktop only has 2011, 2012 and now 2013 libraries, but in the next 3-4 months, I will move the 2011 library to the library external drive.
3. I have found that once I have completed a project, I rarely do additional work on it or the pictures in it.