External Drives for Mac Experiencing Data Loss with Maverick OS -- UPDATED FOR NOVEMBER 6, 2013

This is the fix I used. I went into TERMINAL with the unrecognizable external disk connected. Diskutil repairvolume /dev/disk2s2. That failed. Then, diskutil reformat /dev/disk2s2. Success, then diskutil verifydisk /dev/disk2. Now it shows mounted and it’s back on my screen. May the force be with you!