Disk not Ejected Properly" notifications

That might happen if a bus-powered disk can't get enough power to run properly. Or if the power or data cable is bad or loose. Or if the hub is poorly constructed.

Or if some app still hasn't finished what it was doing -- I try to remember to close any documents or quit such apps before ejecting disks.

Or if some background app still hasn't finished what it was doing. Spotlight or Photos background processes are the most usual suspects -- I usually force-eject such volumes with no ill effects or maybe wait 10-20 seconds before doing so.

Try a different cable and check the connection isn't loose.

Is this only happening when you insert a disk, or did you mean when you eject a disk? If eject, I find that waiting at least 10 seconds before physically detaching a drive helps.