To avoid further confusion, usage of something as a external HD and to sync something, are 2 different things! Dropbox can NOT work as a external storage; it's work is to sync something stored on your HD to the cloud and further to other devices, if any. Of course, you are able to select what you want to be in sync, so to limit local storage requirements. Try don't to mix both they are different, even when seem pretty close!
Dropbox supported charsets for file/folder names are strongly limited, unfortunately! Supported symbols are only those belong to the basic multilingual plane. Any symbol in file/folder name outside mentioned set make the particular file/folder unsyncable! Check your names on the troublesome entries. Any extended symbol, like emoji for example, could be the source of trouble. Let's hope Dropbox will fix this, by you better don't rely on this. 😉