This has happened to me twice with Windows 10 Pro. One was abrand new Dell Vostro which was supplied by Dell with Bitlocker on.
However it should always ask before encrypting any new drive connected. It did not. It also should attempt to give you a key whenever it first applies bitlocker. It did not do this either.
I was able to unscramble this one by plugging it in to the offending Dell, turning off bitlocker and waiting a long time for it to finish the process.
If you are on the correct PC (the one that locked it) surely there should be an easy to find option to turn bitlocker off for an external drive. I couldn't find it and the owner neither wanted or knew that the main drive had been encrypted so just removed the encryption from everything.
The second drive that has been encrypted also happened without warning and I have been unable to track down the offending PC so have now lost the contents of the drive.
This totally unacceptable. External drives and flash drives are regularly used to move data around so Bitlocking them without prompting can never be acceptable.
This needs fixing immediately.