Hello @Jeanie
Please don't format, initialize, erase, or repartition the external drive, as that could make recovery of your files more difficult.
From your description, the first thing to establish is whether the drive was encrypted with BitLocker To Go. Windows uses BitLocker To Go for removable devices such as USB flash drives and external hard drives.
When you connect the drive to your own Windows PC, what exactly does the unlock message say? If it asks for a BitLocker recovery key, please note the Recovery Key ID shown on the prompt. Don't post the actual 48-digit recovery key publicly.
Since this happened after you connected the drive to a school PC, contact the school's IT department. Managed Windows computers can have policies requiring BitLocker protection on removable drives, and recovery information can be stored in the organization's Microsoft Entra ID or Active Directory environment.
Also check whether a recovery key was saved to an account you used on that computer:
- For a personal Microsoft account, check Microsoft's BitLocker recovery-key page.
- If you signed into the school PC using a school account, check the work/school account recovery-key page or ask the school's IT administrator to search for the key using the Recovery Key ID.
Microsoft cannot retrieve, provide, or recreate a lost BitLocker recovery key. If the drive is BitLocker-encrypted, Microsoft doesn't have a master key that can bypass the encryption.
References:
Find your BitLocker recovery key
BitLocker FAQ
If you can post the exact wording of the unlock message (or a screenshot with any personal information and recovery identifiers obscured), we can confirm whether this is BitLocker before you take any further action.
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