USB drives connected to hub disppearing
To troubleshoot this, you could use System Information to inspect the USB tree on all ports and see if you can see the connected hubs.

That might make it easier to diagnose what part of the chain is dropping.

Additionally, the console app can let you search for error messages around the time when the devices go missing. Searching for USB and other disk related messages might turn up some idea if it's an overpower condition where the hardware is shutting down the ports as a preventative measure or due to sustained errors on the USB bus.