@gprovost The files are being shared via netatalk to a Mac. That worked fine until now.
The number of files within a single folder is quite large (up to 13,000 files). The hard drive is formatted with ext4 using the DIR_INDEX option to speed up the file access.
The problem first appeared after switching from one of the four drives from 2 TB to 4 TB. I've copied the files from A to B, so the number of files did not change. But I had to reindex all files within a picture processing software on the Mac (creating thumbs, determine picture properties, etc.).
I've installed the software watchdog, as mentioned above, but that did not help either. If the system freezes a software watchdog does not have any chance to intervene.
After reading about PSU issues here, I disconnected one drive (data + supply voltage) and started the box with only 3 drives. After 10 hours or so of indexing the system did freeze again. I wrote a simple shell script to display temperature and processor load every second. When the system froze the temperature was around 50 °C and the process load slightly above 1.
I also noticed that the "blinking lights" (HD access, SMD LEDs) are all off - even when the box seems to work normally. Is this an indication of something?
Thanks