USB flash drive always runs hot on ASUS RT-ACRH13

Installing and Using OpenWrt
1

I'm running OpenWRT on an ASUS RT-ACRH13 with a USB flash drive as a cheap NAS. I partitioned the flash drive as /dev/sda and sdb. sda is exported as an NFS share and sdb is a luks encrypted local file system (used to rsync files from my Linux desktops/laptops). Both are formatted as ext4. All this works beautifully - except one thing - the flash drive runs extremely hot.

The flash drive stays hot even if I umount both file systems! I'm only reading or writing these file systems for ~3 minutes each day; they're essentially idle. I have tried multiple different flash drives, they all get hot when plugged into the router. None of them get hot (while idle) when plugged into anything else.

I even tried hd-idle. (Yes I know there's nothing spinning to spin down :slight_smile: No help there.

I've been running various 22 and 23 OpenWRT releases on this router, but I'm current as of this week (24.10.0).

I ran OpenWRT on my venerable N-class D-Link DIR-825 from ~2014 until ~3 years ago. (They were still going strong on OpenWRT.) My recollection (always dangerous) is that the flash drive ran cool on the D-Link.

I suppose the heat could be coming from inside the router, not the flash drive itself. I have not disassembled the router - yet. Any suggestions on how to prevent this heat? Note: I'm not interested in how to conduct the heat safely away from the flash drive at this stage. I would like to prevent useless and wasteful heat from cooking my gear.

Edit: corrected the model number

2

It does sound like the heat is coming from the inside. Did you try to touch the USB port without a drive plugged in while the router is running? Or measure it's temperature otherwise. If that's the case, then there's not much you can do.

3

There are two possibilities here:

  • the USB device might just be (mis-)designed to run hot, these things are made to be cheap, for short term usage, many manufacturers don't care about the details (this does vary between different USB sticks)
  • some host devices might radiate heat through the mechanical USB port - or there might by other forms of airflow (fans) pointing in the general direction of the USB port
4

I have a new data point. I stuck a USB-A (M-F) cable in between the router and the flash drive. The flash drive is still slightly warm, but it's nowhere near hot.

I thought I did this experiment 3 years ago - and the outcome was different (flash drive still got hot). Maybe that really was a dud flash drive. (There I go relying on my memory again...)

I can't see a fan in the internal photos:
https://openwrt.org/toh/asus/rt-ac58u#opening_the_case - but there is a ginormous heat sink; and the corner of it is adjacent to the USB connector. Hmmmm... 'I believe that has some significance for our problem' (RPF).

I'm happy to add a cable if my flash drive won't get cooked. IMHO, this is a poor design choice, though...