Balena Etcher is a proper MacOSX thumbdrive writer.
Even from ISO Files 
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Hi
I’ve installed quite a few Mac Minis myself, among them also a 2009 modell.
However, I used the Mac USB DVD Drive, which works well when pressing C at boot - or choosing with ALT (Typical Mac scenarios).
This Network is at a doctors place, there’s still a Mac Mini (2012) running Proxmox here.
If you want to make a bootable USB installer, use BalenaEtcher (Works for almost all stuff, Raspberries, usw.)
Since Apple doesn’t provide any Server functionality, I’ve moved away from Mac Servers. The present “Server” App only provides a very limited MDM (Mobile Device Management), only suitable for Apple stuff. Sure, that works OK, but running a Mac just for that is overkill!
Apple even says, use Linux Stuff, like this (eg CalDAV, CrdDAV servers, DHCPd, and Bind or whatever) but all without GUI? So where’s the Mac advantage? Wasting a good GUI to run console stuff? A Linux box is cheaper.
And sorry, Virtualbox wastes too many CPU cycles on Mac stuff. Proxmox on the same Mac will run circles around Virtualbox!
Besides which: Simple Booting from a CD is a 30+ year old technology, and Macs can boot from CDs without any problems!
I’m quite sure you can get that Mac to boot from a USB cd… 
Try it!
Andy
I’m sure I could. Can you lend me a CD reader/writer? And a CD to write? 
I don’t have a CD other than on an even older Mac mini. A USB stick would be a better / cheaper option if it can work.
Hi David
I think USB boot should work, but it’s not really a bootable disk, it’s an ISO installer.
I had good experiences using BalenaEtcher on several boxes (all without CD drive).
Not all were macs, and getting eg an Acer to boot isn’t easy!
Takes a change in the BIOS, using the boot menu wouldn’t work.
A Mac should boot using either the C key or the ALT key. Keep pressed, until something shows up on the screen… Sometimes using a mac keyboard helps, I’ve had to use funky keyboards on a mac in the past… Not all keys work as they should… (eg a cherry Mac keyboard…)
But so far, all macs were able to boot from USB, so it “should” work.
I DID have one Mac Mini (2011) that had a defective USB controller (Wouldn’t even work in Mac…). I had to resort to a Firewire boot - at least that worked… And finding a firewire CD drive isn’t easy nowadays…
Good luck, and if successful, do post your experiences.
A note: When you do get Proxmox to boot up and start installing, about the 4th screen display a Where To Install menu, where you can choose on which disk to install. Use the options button, and set the Filesystem to XFS.
As to DanBs post earlier: Migrating a running VM from one host to another takes about 90 seconds! Can ANY Mac beat that running VirtualBox? Note: I’m talking LIVE, the VM is accessible and isn’t shut down - and in 90 seconds, it’s running on another box!
Andy
I wonder if the Proxmox’s VMs are stored in the NAS-Syno…
(I need to research how to install Zabbix on my NS)
P.S. I could ask you for a favor: advice or guidance regarding my network configuration on my Proxmox.
Hola Enrique!
I’m running Proxmox servers for 20-30 clients. Actually at about 90%, all VMs are stored in the NAS.
If you look closely, you can even “see” that the NAS is BONDED (2 NICs are connected to the Hub…) this gives more network thruput. (Green Bold & Black dashed)
If possible I have 2 NAS: one for “Shared Storage”, the other for Backups. Since all VMs “live” in the NAS, all Proxmox can access all VMs, this gives you a Live Migration Cluster.
Moving a running VM from one Proxmox to another takes about 90 seconds!
With 3 or more Proxmox, that cluster becomes a full High Availability HA-Cluster.
NethServer “lives” in “DiskImage” a NFS Share on the Synology NAS:
This NAS has a nice setup: 4 SSDs in RAID10 for Proxmox VMstorage, and 4 WD Red in RAID5 for Backups (Not only Proxmox…).:
This NAS has two local USB3 Seagate Backup Hubs with each 8TB, for local Backups of the whole NAS (20 generations!). Additionally, this NAS is synched daily using Synologys tools to an almost identical NAS (All WD Red, no SSDs) at the bosses home. (Off Site Backup & emergency spare NAS for the office…).
The UPS power service is provided by a Raspberry PI for all the Proxmox, NAS and Switches. This is a setup I use at all clients with a UPS.
If you want, send me a PM with your details (hardware, what you’ld like to achieve) and I’ll go over your details and give you some tips how I’d do it, according to Best Practices…
As to installing Zabbix on NethServer: I always use Markus’s Module for NethServer. 
See here and here:
and here:
My 2 cents
Andy!
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Perhaps you can deal with my experiences: Problem configuring NethServer as gateway in Proxmox - #66 by capote
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The reason for this is the creation of a fail-safe cluster with Proxmox. A single system hosting all your VMs is a single point of high risk. If you lose your host, you lose all your servers.
OK, I don’t have a USB thumbdrive and for some reason can’t just nip out and buy one from the local shop! I do have some SD cards and a reader so I’ve used Etcher and put the Proxmox installer on one. But that’s not working. The installer doesn’t appear when I startup holding the option key. Nothing but a white screen if I try to start with the C key held down. Trying to set the startup disk using the Mac’s Preferences doesn’t work. The drive appears but macOS can’t so something to make it the boot drive. Exactly as when trying with a USB HD.
So a USB SD card is just the same as a HD. Thumbdrives are somehow different? If they’re not radically different I give up! They’re all Volumes when plugged in and mounted on a Mac…
Try this one:
Start your mac with the ALT command, wait a couple of minutes on that screen, than plug in your SD Card/Usb flash drive
I used this approach for datarecovery on 2009 iMac with Xubuntu.
Doesn’t want to work for me 
The light on the card reader never comes on so I assume the USB isn’t active or it can’t see the card reader. I’ve also tried starting up via the emergency boot drive in the idea I could then access the startup drive preference and maybe… But all I get there is a blank white screen, no boot at all.
So will someone will tell me a USB thumbdrive is different? If so I’ll get hold of one somehow.
Hi David
I got an iMac 13 2010 yesterday from a client. It has 8 GB RAM and a 500 GB ssd. This one will boot from a USB, but NOT from the built in SD Adapter. An External SD Adapter hooked up to USB works on this one.
My 2 cents
Andy
OK, one last attempt before I try and buy a thumbdrive. What is/was your SD card formatted as? I think my one was FAT 64. I have no idea if this makes any difference but I could try and re-format it and then have another go.
Odd your iMac wouldn’t boot from the built-in SD reader but would from an external card reader. That’s the exact opposite of what I would expect based on my experience so far. And I thought the SD reader was just another port on the USB. But there is more than one USB bus, maybe not all USB ports are the same…
I’ve escaped and bought a USB thumbdrive. £10.
Everything I’ve written above about the SD Card in a card reader applies. I have the exact same results using the thumbdrive as the SC card.
Additionally I’ve noticed:
With the thunbdrive attached and booting the mini using the alt/option key I never get past a white screen.
I tried booting with the alt/option key and once I had the choice of boot drive inserted the thumbdrive. This resulted in the USB keyboard and mouse losing connection.
So, brand new Sandisk USB3 16GB thumbdrive. Used Etcher to copy the Proxmox iso.
I must be doing something basic wrong here. Something so obvious no one is telling me. The format of the thumbdrive maybe?
Hi David
I’ll do a test now on the old Mac I got from a client. I had issues depending on USB speeds, my sticks lying around are still USB2, sometimes too slow…
Since yours are new, that should not be an issue.
What exactly is your Mac modell? (Mini?, Year? CPU & RAM, see About this mac under the apple…)
I’ll report within an hour…
Just downloaded the latest proxmox ISO (6.1) and am “burning” it to USB with Etcher…
Andy
Thanks @Andy_Wismer
Mac mini Late 2009 - 2.53 Ghz Intel Core 2 Duo - 8 GB 1067 Mhz DDR3
I wondered if USB 3 was an issue but its backwards compatible so it can’t be. And why buy a USB 2 drive and only save a quid over a USB3!
No rush, really.
I’m fast… 
I’ve personally had a few of those identical mac minis - almost all the server versions with 2 disks. Even equipped 2 with dual SSDs…
Step one: Booting from USB after keeping ALT (choice) Key pressed:
Step 2: Proxmox installer booted up and ready for action!
As you can see, a freshly downloaded Proxmox 6.1, burned with Etcher on an old 4 GB USB2 Drive (Can’t get 4 GBs for several years now…)…
And the iMac’s ready for installing.
This iMac is a 2010, 3.2 GHz Intel i3 modell, also with 8 GB RAM. The disk has been subsituted for a 500 GB SSD.
If I’m not mistaken, all intel based macs can boot from an external drive, no matter if USB or Firewire / Thunderbolt.
One Gotcha: NEVER use the USB plug on the Apple Keyboard - they’re only 1.1, not enough power and speed. Plug in directly, not via a USB hub.
Proxmox installs after this step…
My 2 cents
Andy
I don’t get as far as the first step which is why I wonder if there’s something obvious I’m doing wrong…
I’ve even plugged the Apple mouse into the back of the mini! Now sitting looking at a white screen where I should have choice of boot drives.
I can boot using the alt key and get the choice as long as I don’t plug in the thumbdrive. So I know how to do that! Then I plug the thumdrive in and after about 10 seconds my keyboard and mouse stop responding. Mouse is connected directly to the mini!
My only other observation is that with the mini booted up as normal and the thumbdrive mounted (its named PVE) it takes FInder a long time to read the contents. It took a minute or so before I could ‘see’ any of the files or folders on the drive. Its as if the Mac really doesn’t like the format or contents of the thumbdrive.
But it works for you…