upgrading an old mac mini

The 2007 Minis are stuck with OSX 10.7 Lion.

You can boot into Recovery Mode using Command + R where you can perform a clean install onto a new drive (best to connect it externally until setup is complete). Or you need to boot from a Retail version of OSX 10.6 Snow Leopard which can still be purchased direct from Apple here. I don't recommend using OSX 10.7 Lion if it will be connected to the Internet as it has a critical unpatched vulnerability.

The 2007 Mini can be upgraded to 4GB of RAM (2x2GB) although OSX may only recognize 3GB. It probably isn't worth the money. It is possible a firmware update included with OSX 10.7 may be required before it will be able to use the 3-4GB.

I really don't recommend putting any money into the 2007 Minis.

The Late 2009 Mini can go up to OSX 10.11 El Capitan.

This Mini can unofficially supports up to 8GB of memory (2x4GB). You may need to have a system firmware update installed which was included with OSX 10.7.5 before being able to use the upgraded memory. You might be able to get by with just the original 4GB of RAM depending on how you will use this computer. I'm not sure it is really cost effective to purchase any memory or hard drive upgrades for the 2009 Mini.

The 2010 Mini can actually go up to OSX 10.13 High Sierra.

This Mini can unofficially support up to 16GB of memory (2x8GB). I would suggest purchasing a single 8GB module and retaining one of the original Apple modules. Again, the Mini may need a system firmware update included with OSX 10.7.5 before being able to use the larger memory module. It might be worthwhile to upgrade this Mini since it still supports are fairly recent OS.

it is highly recommend to purchase your memory only from Crucial or OWC for the best chance of good compatible memory since Macs can be very picky and both vendors guarantee compatibility if you purchase the exact parts recommend on their respective websites.

You have several options for installing a later version of macOS onto the 2009 and 2010 Minis. Probably the best way is to download the macOS installer from the App Store. I would highly recommend creating a bootable macOS USB installer just in case something goes wrong with the upgrade. You can use these instructions to create a bootable macOS USB installer:

https://support.apple.com/en-us/HT201372

John Lockwood wrote:

In addition to what HWTech posted you can also install either Windows or Linux on these Mac mini computers. As an example I have a Mac mini 2012 running Windows 7 via Apple's Boot Camp. (I use it as a Windows Media Centre.)

Depending on what you want to use them for this might extend their useful life.

Linux can be a good option, but the Minis would need to have memory upgrades. To use Linux just to browse the Internet, the computer ideally should have 4GB of RAM, although 3GB is probably sufficient. 2GB of RAM probably won't give a good experience. This is from personal experience. Linux uses about 300MB at boot (best case without customization) and the Mini's GPU uses at least 64MB of RAM so about 400MB of RAM is used by the system before a web browser or other app is opened.

The health of the hard drives is unknown and should be checked before spending money on memory upgrades. If OSX 10.6+ is installed, then DriveDX can be used to check the health of the hard drive. Post the DriveDX reports here using the "Additional Text" icon which looks like a piece of paper.

Before installing Linux, these computers all need to be updated to at least OSX 10.7.5 including all updates so that the Minis include the necessary firmware to handle the extra memory beyond what Apple officially supported originally. It is probably best to download and manually run the firmware updaters just to be safe since firmware updates were not guaranteed to actually be installed back then.

I have just semi-retired Mac mini Late 2009 with 4 GB RAM running OS X 10.11 El Capitan after getting Mac mini 2018.

It is still OK and just a few months ago I have used it for Final Cut Pro to edit HD and even some few minutes long 4K projects.

In light use it is still OK despite the occasional "spinning pizza". OS X 10.6 Snow Leopard would be somewhat snappier but many websites don't work at least with Safari from that era (BTW TenFourFox 7450 opens current websites on my PowerBook G4 from 2004 running OS X 10.5 quite good with enough patience. But I digress...).

Anyway, I have planned to give the Mac mini 2009 a few more years by putting a SSD in it. At the moment I plan to order Samsung 860 EVO 250 GB (57 €) to it and use the old 1 TB spinning HD as a backup disk elsewhere. I also have MacBook 2010 and might do the same with that. (I have already semi-retired the PowerMacintosh 8600/200 from 1998 running OS X 10.3 or Mac OS 9 and the Mac Plus from 1989 running System 6. But I digress...).

BTW, a few years ago I tried some Linux distros with the PowerBook G4 2004 but they weren't any faster so I re-installed OS X 10.3 for a better user experience.

In addition to what HWTech posted you can also install either Windows or Linux on these Mac mini computers. As an example I have a Mac mini 2012 running Windows 7 via Apple's Boot Camp. (I use it as a Windows Media Centre.)

Depending on what you want to use them for this might extend their useful life.