Cannot Back Up External Drives to NAS via Time Machine

External drives plugged in, not in exclusion list, but get “skipped” or “not seen” for some reason when backing up to NAS. Only backing up local Mac drive. External drives are formatted either HFS+ or APFS. TM shared folder set up identically to YouTube suggestions.

So, I can’t get external drives to back up to our DS1821+ for some reason. The local computer backs up fine, but even though the external drive is not in the exclusion list, it does not add it in the backup when it runs. Just acts as though there’s nothing there and finishes the backup successfully without that drive. It’s formatted HFS+, but no clue why it won’t back up to the Synology. On another similar setup for a friend, same thing. It also won’t see her other APFS and one other HFS+ drive we’re trying to get to back up to her DS1522+. This happened on past DSM 7.1 installs and also on the latest DSM 7.2 Update 3, for what it’s worth. All settings are set up just as in this video. All drives are confirmed not to be in the exclusion lists. Any suggestions? (Separately, the only other issues we’ve had were the highly googled Synology+TM issues once backing up more than one Mac where we need to manually kill an SMB connection through DSM and retry a TM backup to get it to work. And that solves our problems there. But still no external drives being backed up).