That’s issue is a really strange one, because we also tested the uTrack with this type of HDD.
Actually I have couple of idea which possible to cause similar issue.
1. If you using too long USB cable there’s a power loss on the cable, in this case the HDD switch to some “power saving” mode, in this case the read and write speed reduced, that’s possible cause similar issue. If you using the stock cable (which was included with the HDD) that shouldnt be a problem.
2. Some HDD have internal protection to avoid from the damage when the HDD moving or falling down. If the music too loud (especially the low frequencies) some HDD push the head to parking position for a while, in this case the data transfer stopped for couple of seconds, that’s possible cause similar issue. Actually I tried to reproduce your issue, I used 3 different HDD for the test (WD My Passport, Toshiba Stor.E partner, Seegate Expansion) the result: the WD was the much more sensitive drive, so I’m pretty sure that caused your issue. For avoid this problem you can put some soft material below the HDD.