My Passport USB3 disappears in Windows 8.1

ddbug,

The WD Passport does feature a built-in sleep timer enabled by default, and once it triggers the disk drive must be unlocked once again. You are able to disable the sleep timer by configuring sleep behavior within WD Drive Utilities.

http://support.wd.com/product/download.asp?groupid=218&sid=171&lang=en