If your external USB hard drive is not recognized by the MobileLite Wireless, you may be experiencing one of the following.
1) The MobileLite Wireless supports capacities up to 2TB. Larger capacity drives are not supported.
2) Supported File Systems are FAT32, exFAT and NTFS. MacOS and Linux File Systems are not supported.
3) Supported Partition Table is Master Boot Record (MBR). Unfortunately,  GUID Partition Table (GPT) or any other Partition type is not supported.
4) If you have a third party external USB enclosure with a standard 2.5-inch HDD drive inside, the power consumption of the HDD may not be supported by the MobileLite Wireless.