The instructions for formatting a USB memory drive (thumb drive) are given on p. 12-26 of the IC-7700 User Manual.
To unmount the USB memory drive after formatting or data transfer, follow the unmounting instructions on p. 12-25 of the manual.
Note that some USB drives are formatted in a proprietary manner, and require the use of a removal tool supplied by the drive vendor to remove this format. If the radio does not recognise the thumb drive, or if the menu locks up when the drive is inserted or the formatting process started, the drive may have a proprietary format. The required tool should be downloadable from the drive vendor's website.
SanDisk is one example which I have encountered. I observed that once I had run the SanDisk tool, the memory drive was left in a format which worked fine in the IC-7700. Further formatting in the radio was not required. With other makes, the tool may leave the drive in an unformatted state, requiring formatting in the radio.
Download the SanDisk Cruzer removal tool here
Last revised: 06/16/18.