it took some time but I was able to resolve my issues. It seems that it was indeed an issue with the power supply of the external 2.5 drive, even when using an active USB hub. I then replaced the drive with an external 3.5 drive and didn't have any issues so far, even with high load. There is also a performance gain with the other drive, so I will not try to make the smaller drive work with the rpi any longer.
Thank you for your help!
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