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?
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.
APPLE INC. SOFTWARE LICENSE AGREEMENT FOR MAC OS X Single Use, Family Pack and Leopard Upgrade Licenses for use on Apple-branded Systems
PLEASE READ THIS SOFTWARE LICENSE AGREEMENT ("LICENSE") CAREFULLY BEFORE USING THE APPLE SOFTWARE. BY USING THE APPLE SOFTWARE, YOU ARE AGREEING TO BE BOUND BY THE TERMS OF THIS LICENSE, UNLESS YOU RETURN THE APPLE SOFTWARE IN ACCORDANCE WITH APPLE’S RETURN POLICY. IF YOU ARE ACCESSING THE APPLE SOFTWARE ELECTRONICALLY, SIGNIFY YOUR AGREEMENT TO BE BOUND BY THE TERMS OF THIS LICENSE BY CLICKING THE "AGREE " BUTTON. IF YOU DO NOT AGREE TO THE TERMS OF THIS LICENSE, DO NOT USE THE APPLE SOFTWARE AND CLICK “DISAGREE”. IF YOU DO NOT AGREE TO THE TERMS OF THE LICENSE, YOU MAY RETURN THE APPLE SOFTWARE WITHIN THE RETURN PERIOD TO THE APPLE STORE OR AUTHORIZED DISTRIBUTOR WHERE YOU OBTAINED IT FOR A REFUND, SUBJECT TO APPLE’S RETURN POLICY FOUND AT http://www.apple.com/legal/sales_policies/. FOR APPLE SOFTWARE INCLUDED WITH YOUR PURCHASE OF HARDWARE, YOU MUST RETURN THE ENTIRE HARDWARE/SOFTWARE PACKAGE IN ORDER TO OBTAIN A REFUND.
IMPORTANT NOTE: This software may be used to reproduce, modify, publish and distribute materials. It is licensed to you only for reproduction, modification, publication and distribution of non-copyrighted materials, materials in which you own the copyright, or materials you are authorized or legally permitted to reproduce, modify, publish or distribute. If you are uncertain about your right to copy, modify, publish or distribute any material, you should contact your legal advisor.
A. Single Use License. Subject to the terms and conditions of this License, unless you have purchased a Family Pack or Upgrade license for the Apple Software, you are granted a limited non-exclusive license to install, use and run one (1) copy of the Apple Software on a single Apple-branded computer at a time. You agree not to install, use or run the Apple Software on any non-Apple-branded computer, or to enable others to do so. This License does not allow the Apple Software to exist on more than one computer at a time, and you may not make the Apple Software available over a network where it could be used by multiple computers at the same time.
B. Family Pack License. If you have purchased a Family Pack license, then subject to the terms and conditions of this License, you are granted a limited non-exclusive license to install, use and run one (1) copy of the Apple Software on up to a maximum of five (5) Apple-branded computers at a time as long as those computers are located in the same household and used by persons who occupy that same household. By "household" we mean a person or persons who share the same housing unit such as a home, apartment, mobile home or condominium, but shall also extend to student members who are primary residents of that household but residing at a separate on-campus location. The Family Pack License does not extend to business or commercial users.
C. Leopard Upgrade Licenses. If you have purchased an Upgrade for Mac OS X Leopard license, then subject to the terms and conditions of this License, you are granted a limited non-exclusive license to install, use and run one (1) copy of the Apple Software on a single Apple-branded computer as long as that computer has a properly licensed copy of Mac OS X Leopard already installed on it. If you have purchased a Family Pack Upgrade for Mac OS X Leopard license, then subject to the terms and conditions of this License, you are granted a limited non-exclusive license to install, use and run one (1) copy of the Apple Software on up to a maximum of five (5) Apple-branded computers at a time as long as those computers are located in the same household (as defined above), are used by persons who occupy that same household, and each such computer has a properly licensed copy of Mac OS X Leopard already installed on it. The Family Pack Upgrade for Mac OS X Leopard License does not extend to business or commercial users.
D. Fonts. Subject to the terms and conditions of this License, you may use the fonts included with the Apple Software to display and print content while running the Apple Software; however, you may only embed fonts in content if that is permitted by the embedding restrictions accompanying the font in question. These embedding restrictions can be found in the Font Book/Preview/Show Font Info panel.
E. Voices. Subject to the terms and conditions of this License, you may use the system voices included in the Apple Software (“System Voices”) (i) while running the Apple Software and (ii) to create your own original content and projects for your personal, non-commercial use. No other use of the System Voices is permitted by this License, including but not limited to the use, reproduction, display, performance, recording, publishing or redistribution of any of the System Voices in a profit, non-profit, public sharing or commercial context.
F. You may make one copy of the Apple Software (excluding the Boot ROM code and other Apple firmware that is embedded or otherwise contained in Apple-branded hardware) in machine-readable form for backup purposes only; provided that the backup copy must include all copyright or other proprietary notices contained on the original. Apple Boot ROM code and firmware is provided only for use on Apple-branded hardware and you may not copy, modify or redistribute the Apple Boot ROM code or firmware, or any portions thereof.
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H. Certain components of the Apple Software, and third party open source programs included with the Apple Software, have been or may be made available by Apple on its Open Source web site (http://www.opensource.apple.com/) (collectively the "Open-Sourced Components"). You may modify or replace only these Open-Sourced Components; provided that: (i) the resultant modified Apple Software is used, in place of the unmodified Apple Software, on a single Apple-branded computer; and (ii) you otherwise comply with the terms of this License and any applicable licensing terms governing use of the Open-Sourced Components. Apple is not obligated to provide any updates, maintenance, warranty, technical or other support, or services for the resultant modified Apple Software.
You expressly acknowledge that if failure or damage to Apple hardware results from modification of the Open-Sourced Components of the Apple Software, such failure or damage is excluded from the terms of the Apple hardware warranty.
I. You may not and you agree not to, or to enable others to, copy (except as expressly permitted by this License), decompile, reverse engineer, disassemble, attempt to derive the source code of, decrypt, modify, or create derivative works of the Apple Software or any services provided by the Apple Software, or any part thereof (except as and only to the extent any foregoing restriction is prohibited by applicable law or to the extent as may be permitted by licensing terms governing use of Open-Sourced Components). You agree to use the Apple Software and the Services (as defined in Section 5 below) in compliance with all applicable laws, including local laws of the country or region in which you reside or in which you download or use the Apple Software and Services.
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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