Oh I am well aware about the differences between "how it should be" and what is actually the case when you're a small developer.

For this kind of issue one could use VM's where you encrypt the VM with the live customer data. Another option would be to use cloud.

But yes.. there's also nothing wrong with having more than one machine to physically separate the systems and thus data. Bonus on that one is that you can shut it down if not needed and as such it won't be accessible over the network unless switched on.

Personally -as a small developer myself- I use all of the above.