I come from a Mibox 3 that let me download files from my FTP server to the externally connected HDD, which was great. Now, the new Mi Box S does not let you do that anymore, and I tried some of the workarounds I found Googling but to no avail.
Tried to format a 500gb HDD to be set up as internal storage, but so far that has led to the box freezing up on me twice now. Anything I am missing?
Also, if I format 1 HDD to be recognized as internal storage, can I use a USB hub and have another HDD as external storage?
Out of box, you can't write to USB drive attached to Mi Box as removable storage in Oreo. You can attach multiple drives with a USB hub (only one can be formatted as device storage). One thing you need to watch out is power consumption, especially with mechanical drives. I have two USB sticks (one formatted as device storage and another as removable storage for sharing media & apk files occasionally), a wired PS4 controller, and a USB key for wireless keyboard/mouse combo attached to a USB hub without its own power source.
But 3rd-party OTG driver supports limited write access to USB drives. I say "limited" because you can only write to USB drive in specific file managers, not in whatever apps you want. Here are a few apps with 3rd-part OTG drivers that I have tested on Mi Box. Look for "custom USB OTG driver" option in their settings. Once you enable it, next time when you attach a USB drive there will be a popup asking whether you want to mount it with that app (try to enable only one custom OTG driver a time).
X-plore ( https://play.google.com/store/apps/details?id=com.lonelycatgames.Xplore&hl=en_US ) : FAT32 (read/write), exFAT (read-only)
MiXplorer ( https://labs.xda-developers.com/store/app/com.mixplorer ) : FAT32 (read/write), exFAT (read/write), NTFS (read-only)
Microsoft exFAT/NTFS for USB by Paragon Software with Total Commander ( https://play.google.com/store/apps/details?id=com.paragon.tcplugins_ntfs_ro&hl=en_US ) : FAT32 (read/write), NTFS (read/write), exFAT is not supported on Android TV devices (they claim Microsoft didn't give them the license)
Finally, a word of caution, there's always the risk of data corruption when writing to a USB drive with 3rd-party OTG driver. So it's better not to use 3rd-party OTG driver on USB drive that carries very important data (unless they are already fully backed-up).