The only thing that worried me was external HDDs that rely on USB power. I'm happy to say that it seems to work in my case with a USB 3.0 2TB WD My Passport. Transfer speed is not that great. about 20-25MB/sec to the internal drive and 10-12MB/sec to the 64GB SCDX MicroSD that was installed but that was to be expected.
Generally yes. I attached a Corded Keyboard via an OTG micro-USB adapter and it was recognized instantly. It immediately worked with a wifi dongle for my wireless Keyboard and Mouse too. As well as with my 32GB Flash drive. No issues at all. When it comes to external Hard Drives or DVD players, that depends on how much power they need. If they have external power, then they should work perfectly OK. (some DVD players require power from 2 USB, obviously not an option with this Tablet).
I tried it with a Toshiba 2.5" portable hard disk and it did power it up but apparently not enough to function properly, it wouldn't read the disk. A powered USB hub would be needed to ensure this would work.