USB sticks are great to use for transferring your work between devices but not great as a back up. They tend to get easily damaged or corrupted and some that may work with Windows may not work with Macs or vice versa. Google Docs and Google Drive are the best methods to use for working and saving work.
When starting work in a new document, always make sure it has a unique name so it doesn't overwrite other files. Do not open files if you are using a USB stick, copy the files first to either Google drive or your network drive and make sure you open and edit the copied file. We cannot recover corrupt files from USB Sticks.