Reboot during frequent HD I/O

My WRT1900ACS keeps rebooting when there are frequent HD I/O (particularly, rm or chmod). I have been using DavidC502's LEDE build, and I returned the 1900ACS because I thought it was a hardware defect. Now I got a new 1900ACS and installed his latest build (Lede Reboot SNAPSHOT r3988-f66e7cb), and the issue occurs again. Does anyone has some clues of it?

I have been using two different USB hard drive with HFS+ volumes. The reboot issue typically occurs whenever I rm or chmod a large amount of files on either drives.