How to access encrypted LaCie SSD for reformatting after Time Machine removal?

Thomas Claburn wrote:

It's an Apple adapter. The device gets power from the laptop. I hadn't been a problem previously -- it was slow but it could complete backups. I have a separate one of the same model that I to backup a different laptop running macOS Sonoma (with the same adapter) and it still chugs along.

Getting power through that adaptor is not supported by Apple, based on its support page for that adaptor. So that means it might or might not work. The fact that it worked before doesn't mean it will work in the future. Or it might have worked with inadequate margin and now that the hardware has aged, the drive might not be getting enough power anymore. Or the cable or adaptor might have worn out.

I am guessing that this drive (as well as its cables) is more than 10 years old. The fact that it is detected in system information but cannot be seen in Disk Utility (can you even see it but it is shown as "unmounted" in Disk Utility?) means it cannot be used, but even if you could somehow figure out a way, would you rely on that drive for anything now? Certainly not for backups, which must be bullet proof. Something significant has gone wrong with that drive. Modern drives are very inexpensive, you should get a reliable one, anything else is "penny-wise and pound-foolish."

You said it worked but "was slow." What does that mean? Thunderbolt 2 supports data transfer of at least 1.5 Gbps, which is about 200 MB/s. Perhaps in real life use one should see at least 50% of that. What speeds were you seeing? USB-C is also quite fast, by the way. If your transfer rates were "slow," that is also a red flag.

I would recycle the drive and get a new, fast, reliable one so you can do a reliable backup asap. Consider replacing the other one as well.

Thunderbolt 2 cable ... that must be a very old drive. What type of adaptor are you using? Is the device getting power from its own power supply? Are you certain that the combination of Thunderbolt 2 cable and adaptor is compatible with Sequoia? Were you using it with 15.3 before and it was ok?

Do you have any LACIE firmware or software tools active or installed? If so, are you certain that they are compatible with 15.3? Was the drive encrypted using Apple MacOS or using LACIE tools?

Thunderbolt 2 would mean that the drive must be 10 years old or more. Mechanical drives sometimes slowly start to require more power as they wear out or the mechanical spinning parts start to degrade and they don't reach a speed where a computer can access them. They can fail at any time. The Thunderbolt 2 cable is also likely old and could have failed. Or the adaptor. If this drive is really that old, I would be thankful that these issues showed up when you aren't actively using it versus depending on it for something and losing files. I would replace the drive rather than try to revive a Thunderbolt 2 drive. Macs are currently on Thunderbolt 4. I think Thunderbolt 1 and 2 hardware is only available now on sources like Ebay.