Upgraded to Sierra recently. I have 2 external hard drives, both Western Digital, 4 and 8gb. About every few days I'll open one up and there are no files, the bottom says 0 items. So I restart my iMac and all the files are back as normal and it's okay for a few days. I've even opened them up to see the list of files disappear in front of my eyes. Any ideas? Thank you