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?
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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.
The catch with Apple is that you cannot install older OS X/macOS your Mac was shipped originally with. Your MacBook Air came with OS X Yosemite (10.10.2). And you won't support OS X versions older than that. The 10.6 Snow Leopard was released a decade ago and that's why you can't install it on your Air.
You should not be able to boot the Snow Leopard stored on external drive using 2015 MacBook Air.
It might be possible to boot the external drive using a Virtual Box machine running on a 2015 MacBook Air.
However before attempting to use a virtual machine, have you considered installing the crucial software on the 2015 MacBook Air? Most of the software I used, which worked with Snow Leopard, still works with High Sierra. This software original was in the form of DVD's. To install on my 2013 iMac (which does not have an optical drive), I used one of the following solutions.
DMGfile. Transferred the file to the 2013 iMac and install the software.
Another possibility would be to boot to OS X Recovery over the internet and upgrade Snow Leopard to the OS X that was originally installed on your iMac. This should be either Yosemite or El Capitan. This option might require Snow Leopard be version 10.6.8.
There is some controversy regarding whether running Snow Leopard in a VirtualBox virtual machine would violate the SLA. To be clear, I intend for the OP to leave Snow Leopard on the external drive. VirtualBox would then be used to boot from the physical external drive. Here still is only one copy of Snow Leopard installed on the external drive. Snow Leopard will boot and execute using Apple hardware.
Basically the SLA requires you run Snow Leopard on only one Apple machine at a time. There is no requirement that the machine be physical or virtual, as long as the physical hardware is Apple.
I know the SLA for Snow Leopard does not explicitly refer to virtual machines and the SLA for later versions of OS X and macOS do make such references. But, these later references are in addition to the requirements for the first installation. In other words, the later SLA's allow for simultaneous executions of the same licensed operating system, as long as the additional executions are in virtual machines.
For example, the SLA for Mavericks contains the word directly in 2.B.(i) which would indicate a non-virtual execution, but 2.B.(iii) then allows for virtual executions. The SLA for Snow Leopard simply states a single Apple-branded computer. There is nothing in the SLA stating Snow Leopard can not be run virtually. There is only the requirement for the purchase of a SLA for each actual or virtual simultaneous execution of Snow Leopard.
Note: One strange thing about the Snow Leopard SLA: the word snow never appears.
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Note: I could not include the rest of the SLA do to the answer limit of 30,000 characters.
EA0560 Rev. 7-15-09