In most cases, it is normal for an external USB device to get warm or hot to the touch since the exterior shell helps cool the internal components by emitting heat into the air. Some of the factors of a USB external device getting warm or hot:
Material: Metal flash drives and readers tend to get warmer while in use than other types of USB flash devices. This is simply because metal absorbs heat much more easily than materials like plastic do.
Environmental Temperature: If the host device or surrounding temperature is high, the drive's cooling capability decreases. USB drives often become warmer when used in hot environments or with warm devices.
File Transfers: Transferring large files or a high volume of files can raise the drive's temperature. Large files take longer to transfer, requiring more energy, thus generating more heat. Quick consecutive file transfers have the same effect as transferring large files, increasing the drive's temperature. During data transfer, the drive heats up internally and releases heat through its casing.
Note: None of the above listed factors impact the functionality of the USB device. After the data transfer is complete, wait 30-60 seconds before removing the USB device from the host machine. This should provide enough time for the temperature of the USB device to cool.