Different brands of USB flash drives, external hard drives, and data cables can not fully comply with standards, which can result in recognition issues.
To resolve this, please try the following steps:
1. Reinsert the Type-C connector in reverse to ensure a proper connection;
2. If the cable is damaged or incompatible, using a different, high-quality cable could help establish a proper connection;
3. Ensure that your external storage device is compatible with the Xiaomi 15 and formatted correctly.