I have been running an umbrel for awhile. It may be user error, but it doesn't seem to stay online for more than a couple of days at a time. On a pi4.
Discussion
I don't know your set up, so I'll just throw out a thought. If you have an external HDD is it plugged into the USB 3.0 port? Or is it plugged into one of the 2.0 USB ports? The 3.0 provides more power to the HDD. Also, take a look at the power requirements of the HDD. And evaluate your power supply to the pi.