I have an external SSD containing a bootable installation of Mac OS X Snow Leopard 10.6. It also contain some software crucial to my use.
Can I boot into it using my 2015 13" MacBook Air?
MacOS – Boot into Mac OS X Snow Leopard 10.6 on a 2015 MacBook Air
bootmacos
Related Solutions
There should be a point—after you choose the language, but before it starts trying to install the OS—where you see a Utilities option in the menu bar.
Do you get to this point? If so, what happens when you choose Disk Utility?
There's more information about this at Using Disk Utility to Repair a Disk (University of Wisconsin).
OK, now it's a different question, so here's my different answer:
If repairing your main partition is what you want to accomplish, why not just run Disk Utility off the 10.5 system disk?
You'll need to make your own bootable DVD/USB of the Lion installer to do so. This can be made from the AppStore download and is documented well by OSXDaily.
Lion USB Installer
Lion DVD Installer
Then hold ALT at system boot (before chime & Apple logo) and select your install volume.
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Best Answer
Unfortunately you can't.
OS X Yosemite 10.10.2 is the earliest OS supported on your 2015 MacBook Air. Mac OS X 10.6 Snow Leopard is too archaic (released in 2009) and you won't be able to boot into it on your 2015 MacBook Air.