using an external hard drive for macbook air

This is a well known limitation on how far you can take it. It doesn’t always work (Ex: Core Duo original to a Core 2 Duo/Core i series with a dual video subsystem may not boot), but it usually does in most situations well enough to get your data and wipe the system. The reason this generally ends up working is Max OS X is more receptive to total system changes compared to Windows. It typically works if the system is similar, but if it’s too radically different you may need to take a backup and start over.

Yes, but there are no guarantees the system will boot with your old configuration or operating system. If Apple won't get a more consumer-friendly policy, then they should restock inadequate computers people buy because they can't test first. Your best option may be to purchase the computer you think will work, transfer your data from your old Mac/Time Machine using Migration Assistant, test your apps, and take your new computer back immediately if it doesn't satisfy your need. If you're dealing directly with the Apple Store, you have to 'buy before you try'. This is irritating since they stopped installing pro apps like Photoshop on the demo systems.

This means that you can't walk in with an external drive and boot the computer into your own setup for testing purposes, or with an older Mac in target drive mode. They're spooked by malware, even though they're likely to be booting into guest accounts which could be wiped at shutdown. Apple Store note: For the last few years (since 2015 at least, and maybe for a long time before), Apple Store staff won't let you to connect an external drive to a demo computer.

Using an external hard drive for macbook air