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It’s very unfortunate but thankfully there are 3rd party replacement SSDs now that are cheaper and faster than Apples offerings. Downside is you need a 2nd Mac to restore the Mini after you’ve completed the upgrade.
I thought there was a Windows or Linux Utility now, could be wrong though.
 
It's annoying having to deal with "System Data" on the internal 256 GB SSD taking up 136 GB of space. I don't know what's going on but that's what I've been dealing with here, keeping the internal as the boot device and using the external for data storage which is likely what most people would do. Apple really needs to fix storage usage in MacOS if it's not going to offer 512 GB as base. (Or maybe frustrating people into expensive upgrades is a feature and not a bug for Apple).
There should be an option to not install other languages.
There's also a lot of other stuff that consumes considerable space live screensavers and voices, all of this content should be able to be deleted, shame the system is so locked down, there should be a way to get read/write access to the system partition.
 
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