Question about TechniSat Technician K4 ISIO

I have been using the K4 ISIO for a few weeks now and am basically very satisfied. Unfortunately, I have a problem with recording. I use a normal external 1TB HDD hard drive from WD. Every now and then the recording cannot be played back. In other words, it can be played back, but the screen is either black or shows a still image. The recorded programme is displayed normally in the overview and I can in principle fast-forward, pause, play normally etc. for the entire duration of the recording. Is there an error with the recording? In 90% of cases everything works normally, but sometimes (possibly when recording two programmes at the same time) unfortunately not. Should I use an SSD hard drive? Or could it be because I have inserted a splitter in between the input signal (as if I had a second receiver)?