My new macbook air m1 just ruined my external ssd?

FYI, it's more likely that the drive itself is the problem.

There are massive problems with Sandisk Extreme and Extreme Pro external SSDs as well as problems reported with other Sandisk SSD models.

I got bit by this myself a while ago.

Return the drive and buy something else.

The problem is not your Mac. The problem is the drive. Stay away from Sandisk at all costs. They claim they have fixed the problem, but there is now evidence that the problem is a hardware design flaw, not firmware. They have not recalled them as they should have, they simply wait for them to fail in hopes that most people will blame it on something else and not file a claim for the defective drive.

https://www.digitaltrends.com/computing/sandisk-ssd-firmware-fix-fails-hardware-havoc-revealed/

https://www.techspot.com/news/100880-sandisk-defective-ssds-affected-major-hardware-flaws-data.html

https://www.msn.com/en-us/news/technology/sandisk-extreme-pro-failures-result-from-design-flaw-says-researcher/ar-AA1jNQbD

This is just a small sampling of recent news articles.

I don't know how the reformatting occurred. This is the first time I've heard of this happening. I do have a couple of thoughts which may be useful.

• Many of us here have found that when an external disk is purchased, the vendor provides a so-called maintenance app. They virtually always cause problems when used to format the disk for a Mac. Did you use such a maintenance app?
• These disks should only be reformatted using Disk Utility.
• When using Disk Utility to check and repair a disk which is reported to have a problem, you may have to run D.U. several times before you get an ok indication. Some people do not know this.
• External disks do fail, even new ones. Not often but it happens.

I went with a Samsung T7 Shield.

It's not as fast as the Extreme Pro was, but the extra speed doesn't really mean much when it spontaneously loses all your data.

There are many fine disks ou there. For general use I have had good success with LaCie disks and they are reasonably priced.