HTC One User Manual page 203

About storage encryption
On the phone storage, you can encrypt data such as your accounts,
settings, downloaded apps and their data, media, and other files. On a
storage card, you can only encrypt files.
Set a lock screen PIN or password before you start encryption. Fully
charge the battery first, or keep HTC One connected to the power adapter
during encryption. This process may take about an hour or so.
You cannot undo encryption on the phone storage. If you want to return to
an unencrypted phone, you must perform a factory reset.
Once you enable storage card encryption, only new files added after
encryption will be encrypted. Files already on the card will not be
encrypted. Also, storage card encryption doesn't reformat or erase your
card. Encrypted files on the storage card can only be opened by the phone
that encrypted them.
How to encrypt
1. Go to Settings, and then tap Storage.
2. Tap Phone storage encryption. Or, tap Encrypt SD card content.
3. Tap Next.
4. Enter the lock screen PIN or password, and tap Next.
5. Tap Enable encryption.
During phone storage encryption, the phone may restart a few times.
When storage encryption is complete, enter your PIN or password.
Storage 203