A USB storage device connected to an AirPort Extreme (802.11n) base station may not appear in AirPort Disk Utility, or may not appear with the expected name. This can happen if the device's volume is named with high ASCII characters or 2-byte characters.
Disconnect the volume, connect it to a computer, and rename the storage volume so it has no high ASCII or 2-byte characters. Then, reconnect it to the base station.
Two-byte characters are commonly used in Japanese, Chinese, and Korean fonts. High ASCII characters are "extended ASCII" characters that you typically type using the Option key. They are listed in table II of this webpage: (http://www.cdrummond.qc.ca/cegep/informat/Professeurs/Alain/files/ascii.htm).
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hard drive