I'm on the hunt for an easy to use app that can keep multiple folders in sync. (Example use: Keep selected folders on an external hard drive in sync with folders on google drive or vice versa.) Trying to avoid constantly dragging and dropping to update latest files.
- I use Syncthing for exactly this. Keeping 2 folders in sync - one of my desktop and one on my laptop. I used to use dropbox for this, but didn't like the overhead and keeping all my files in the cloud. With Syncthing there is no central point and it has a nice web based gui. *Update* Oeps I just saw the last bit of your question which seems to suggest your looking to sync google drive. In that case Syncthing won't do that for you.
- Insync works pretty well with Google drive, adding multiple google accounts is a breeze (e.g., one work account and one personal account). It does 2 way sync and recovery from errors are pretty robust. You can also pause it any time from the menu bar. Local folders for sync can be on your hard drive or a external drive and it will sync just fine.