Old 2013 Macbook Air - Install works, but won't boot afterwards

[1337haXXor]

I tried copying the refind.deb file (alternate installation method) into chroot, which worked, but I can't install it. I get a "system user got removed" error. And if I use sudo, I get an error saying "you do not exist in the passwd database.

[baldyeti]

you probably need to run "apt-get update" first, refind is definitely available for MX21

and there is no need for sudo with chroot, since you are already the "root" admin user (at the "chroot>" prompt)

[fehlix]

After installing, from the rescue menu, it does not detect the install.

Not sure what this means. There is nothing to install from the rescue menu.
On MX LiveUSB the rescue menu does have two entries to select for searching for GRUB boot loader and another one for GRUB menus. Have you tried both? In case bothe entris won't find anything, may be the ssd is not detected by GRUB.
You still can use rEFInd USB-key, in case rEFInd might detect better the SSD drive and and will display the efiGRUB loader to select.

[1337haXXor]

Okay, I feel like we're getting warmer. Running apt update/upgrade after chroot indeed did the trick, didn't think about that. I've installed rEFInd, but the issue persists. The output of efibootmgr, which lists bootorder is:

0001, 0000, 0080
0000 = MX21
0001 = rEFInd Boot Manager
0080 = Mac OS X

Running it verbose, shows MX21 and rEFInd's boots on the EFI partition of the correct UUID hard drive. The Mac one shows on a different drive, but that's fine, because I don't need it. But the path of the MX and rEFInd ones are \EFI\xxxxx, whereas the Mac one is \Volume\ESP\efi\refind\xxxxxxxxxxxx. I understand that the boot has to be on the ESP for Mac to read it, correct?

EDIT:

After installing, from the rescue menu, it does not detect the install.

Not sure what this means. There is nothing to install from the rescue menu.

Sorry, I mean after having installed MX to the SSD successfully, using the boot rescue menu in grub from the USB does not detect a grub nor boot.

[m_pav]

Mac use a modified freebsd, we use Debain, they are 2 rather different animals that speak different languages, so it means nothing if mac says \Volumes when Debian says / It's just a different way of identifying the same thing, and we all know Mac operates on a level of near infamy bordering on insanity
Try force booting MX by starting with a Live-USB, then going into the advanced menu > boot options boot from HD. Then return to the main menu and hit Return. Not sure if it will take you directly to what it finds in the efi partition on a mac or not, but it's worth a try. If successful, you can update, then reinstall refind and see how that goes.

[1337haXXor]

Man, tell me about it. This is insane the amount of work it's taking simply to boot something else on a physical device. I'm not a Mac person, and really I'm just trying to revive this old laptop for a friend. So if it doesn't all work out, I guess I can just run with the original Mac install, though it's a bit slow and definitely quite outdated.

Anyway, yeah I tried adding boot from HD in the advanced options, but it didn't work. After enabling that option trying to boot with the USB out gives me:

error: invalid buffer alignment 319951120
error: you need to load the kernel first.

Booting with the USB plugged in gives me a FATAL ERROR, lol.

Cound not find file antiX/linuxfs
Searched devices: /dev/nvme0n1p3 /dev/xxxxx (all my partitions)
Searched types: hd
All block devices:

• Please contact BitJam at https://mxlinux.org
  
  Then options to power off or reboot.
  
  Side note, I really appreciate all of you guys trying to help out! I'm glad it's not crucial that I get this working, but I will say this is definitely one of the most positive and approachable communities I've participated in, haha.

[fehlix]

Booting with the USB plugged in gives me a FATAL ERROR, lol.

Cound not find file antiX/linuxfs
Searched devices: /dev/nvme0n1p3 /dev/xxxxx (all my partitions)
Searched types: hd
All block devices:

Seems from=hd would ingore the LiveUSB, better select "from=all" to boot the LiveUSB.
Also, not sure whether you answered my questions, which of the two GRUB search entries efiGRUB loader or GRUB menu on the MXLiveUSB-Rescue menu, did not find anything and if found a GRUB entry would it boot or gives further error messages.

Lastly, suggest to get the rEFind liveISO/USB, which may help to boot.
Which is mentioned on DebianWiki
Boot_your_computer_with_the_Refind_media
and pointing to Getting rEFInd
where you get an USB- and a CD-image, which may (or not) help booting EFI (and may work on iMacs too).
Good luck.

[1337haXXor]

Both of the grub entries found nothing, so I couldn't boot anything. It would indeed appear grub is not detecting the drive, as it only shows HD,2, which I believe is the flash drive?

Either way, no worries. I formally give up, haha. I have to get the computer back to the owner. Figured I'd update here, for anyone from the future showing up, hoping to solve this. It seems like for this specific situation, or anyone else in it, your best (or only) option is to boot MX (or any other OS) via USB. Thanks for all your help, everyone!