Flaky external SSD via FireWire

H AND J wrote:

[...]Question: Can the OS be reinstalled w/o erasing data?

Yes, the OS can be reinstalled w/o erasing your user data. However, it is always strongly suggested that you create some sort of backup of your stuff before proceeding with the process.

How to reinstall macOS - Apple Support

Boot the Mac into recovery using the Command-R key combination. This will (should) offer to reinstall the current version of the most recently installed OS on your Mac mini.

The 2012 Mac mini supports USB 3.0 which is faster than FW400 and just as fast FW800.

What I would do is put the San Disk SATA SSD into a USB 3.0 enclosure.

FWIW I have 5 of the following tool free Sabrent enclosures and they work with all of my old and new Mac's.

for example: https://www.amazon.com/Sabrent-Tool-free-Enclosure-Optimized-EC-UASP/dp/B00OJ3UJ2S?th=1

H AND J wrote:

Thanks for your reply!

Unfortunately, lack funds for newer enclosure/dock.

Although FireWire 800 is certainly faster than USB 2.0, my experience with using a FireWire 800 SSD on an old Mac suggests that FireWire 800 is a bottleneck limiting the performance you can get out of a SATA SSD.

You can get tool-free USB 3.0 enclosures for 2.5" SATA drives for as little as $7 – $16 on Amazon. I'd suggest looking for one whose description says that it supports UASP ("USB-Attached SCSI Protocol") – since this is a rather common feature, and one that may help to improve performance.

What does the slash circle mean? No System Found?

If your Mac starts up to a circle with a line through it - Apple Support

"A prohibitory symbol, which looks like a circle with a line or slash through it, means that your startup disk contains a Mac operating system, but it's not a version or build of macOS that your Mac can use."

The system may have gotten damaged (corrupted) during the forced restarts and forced shutdowns. Whatever happened, the Mac no longer sees it as being valid to boot from.

You're welcome.

What is the make and model of the Dock...?