Moving hard drives from one Mac to another I have Mac Mini server with one 500Gb HDD (with system) and another 1 Tb SSD (with data). The computer shows no sign of life, and my Time Capsule, unfortunately, is also dead. So, I have bought the same model of Mac Mini, but unfortunately with lower disks capacity (250 and 750 Gb).
So the question is, can I just physically replace the disks in working Mac by the other from my old not working Mac? Will they work or there is some kind of protection that will block the system? Are there any pitfalls?
To save any potential issues with firmware changes over the years, make sure the new Mac is running the same or newer macOS as the old one was.

Mojave, for instance installed a firmware update to many machines at the time. I'm not certain this would cause any real issues if it wasn't present, but it would save any uncertainty.

Once you've done that, then yes - as the Mini Server is pre-T2 security - you should be able to simply swap out the drives with no ill-effects whatsoever.

if you have an external UDB dock that would take your system HD &amp; the new machine's 750GB SSD, I'd consider using something like Carbon Copy Cloner to clone the HD to SSD, for the added speed.