From apple discussions:

(note this relates to time machine as I wanted to decrpyt since I’m moving to linux. Once on linux, this script seems to be necessary)

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I too was under the mistaken belief that I would be able to turn off encryption the same place that I turned it on. This cannot be done from within Time Machine (at least not in OS X 10.9) – you need to do it from within the Disk Utility:

Open Disk Utility
Select the backup disk
File > Turn Off Encryption…
Enter the password for your encrypted disk

This will turn off encryption for your backup disk, and decrypt the data that is currently encrypted.

Mario Loria is a builder of diverse infrastructure with modern workloads on both bare-metal and cloud platforms. He's traversed roles in system administration, network engineering, and DevOps. You can learn more about him here.