2015 MacBook Air (13" - 256 SSD, 8GB RAM) Boot problems This is my Wife's Mac, and she has been complaining of general slowness for a while now. And I confirmed it and did the normal things: Safe Mode, clean caches, check the disk for errors, run the hardware diagnostics, zap the PRAM/NVRAM. The result: nothing found, still slow.
So time for a new Mac for her (a new MBA) and I scrub the drive on the old MBA clean of all data with disk utility, run first aid and (try to) reinstall macOS (Mojave)in the hopes that a clean reinstall will do the trick.
Internet Recovery mode errors out when trying to install Mojave from Apple's servers (and a 30 minute boot), so I make a Mojave installer on a USB-3 flash drive and boot from that.
It takes almost an hour to boot the flash drive and even then the progress bar completes but nothing happens. So I built another installer and verified this one will boot an older Mac Mini I have. Works fine, but on the MBA same as before: an hour to complete booting but I never get the Install MacOS Mojave screen. It sits, completed on the black screen with a filled progress bar.
So Diagnostics finds nothing, disk first aid finds nothing, zapping the PRAM/NVRAM does nothing, and now I have an old MBA with no OS that won't install a new OS and stumped as to what is going on. Not sure it is worth taking to Apple for repairs as repairs would likely cost more than the Mac is worth.
Any suggestions or even ideas what it might be?
If your installer/boot media works fine on a different Mac but fails on your MacBook Air, the issue is likely hardware (logic board).
You don't need a drive to load the installer.  If you boot from USB or boot from the network (Internet), it is caching the installer in memory.  If you're to run diskutil list command while the installer is running, you'll find many &quot;disks&quot; from 0 to 20-something.  These are the RAM disks and this is normal.
What you've done by booting both Internet Recovery and USB installer is bypass a potential failure point - the internal storage device.  The fact that the problem still exists  using both of those methods indicates you have logic board issue (possibly CPU or memory).  By booting from Internet, you remove both the USB bus and the internal storage from the equation.  Process of elimination says there's nothing more to bypass - the problem is with your main board.
Apple (service) isn't going to do anything that you've already done.  Apple won't repair this - they will quote you for a logic board replacement.  This normally runs about $5-700 USD.  At that price point, I'd go for a new machine.
Also Sell that one as salvage and recoup some money.  The screen is good, the battery, case, keyboard, trackpad, etc. can all be used for repair parts by a shop.  Even the logic board can be salvaged for individual components (the SMC chip is in high demand).