If you're using BioStar 2.6, you would have noticed this popup appear every time you connect a device:
The warning message says "Change the device's key to match the server's data encryption key. All user data in the device will be removed after the key change."
It sounds like all the users in the device will be deleted, but this only applies if you're using the secure communication with device (KB article link) feature.
If you're not using the feature, you can ignore the warning message.
The warning message means that if a device is connected to the server for the first time after secure communication with device feature is turned on, all the users will be deleted in the device and synced with the server again as the encryption key is applied.
This warning message will be changed in BioStar 2.6.1 for a more clear message saying:
"When the encryption key value between the server and the device is not the same, the device's encryption key is changed according to the server, and all user data in the device will be deleted."
This message will only appear when secure communication with device feature is on.