Sync to external hard drive problems

Have just begun using DTP, and as my database is very large (about 10GB), I use an external hard drive for sync, as DropBox is impossibly slow. I set my databases to automatically sync on opening and closing, but yesterday, I forgot to plug in the external hard drive first. Now the database refuses to sync to that hard drive, saying I do not have the permissions to do so, even though it is plugged in. How do I reset this?