I would love to try Yosemite PB2 on a retina MacbookPro but i dont know of anybody who has one. Would it be possible to install the PB 2 on my machine (macbook air 2014) on an external harddrive and then boot from it on another machine by just plugging in the HDD?
That is what I have done. It was easy to do but takes longer to install & boot etc. I use the external drive to test Yosemite on my Imac and Macbook.
I believe the external drive needs to be formatted as mac os journaled using disk utility. Also I think the drive/partition needs to be set to Guide (also in disk utility). Doing this will wipe anything already on the external drive. When installing Yosemite you then need to make sure you choose the external drive and not accidentally install it on main hard drive like someone appears to have done on this forum. Always best to have a backup though.
To boot from the external drive hold down the alt/option key when booting computer and chose the yosemite drive. If this does not work go to System Preferences > Startup Disk > (select yosemite drive & click restart).
Google also has a few instruction on how to do all this if need be.