I will look for an SSD then!
I just measured all power consumptions with and without USB disk connected.
With the USB connected the consumption is 1.3 -1.5 W higher, than without.
But I got mostly pretty stable measurements with only little fluctuations depending on streaming provider and source.
For example Amazon Music: 6,7-6,8W
Spotify Connect: 6,9-7.1W
and so on... (with a delta of only 0,1W within each source)
Playing back from USB on the other hand -> the consumption varies a lot 7,5 - 8,1 W. with a delta of 0,6W
=> maybe this points also to a power problem. I dont know...
No issues with indexing at all, though.
Never the less, I will look out for an SSD.