Sunday, April 3, 2016

Tale of the Loose Cable

Friday I used my laptop (Acer Aspire V5-131, Linux Mint 17.3) at a coffee shop with no problems. Saturday it wouldn't boot. The specific error message was:
Broadcom UNDI PXE2.1 v15.0.1
<blah blah blah>
PXE-E61: Media test failure, check cable
PXE-M0F: Exiting Broadcom PXE ROM
No bootable device -- insert boot disk and press any key
The message suggests that either the hard drive was down, the data cable to the hard drive was loose (or broken), or the hard drive was somehow no longer bootable. Searching online essentially confirmed that.

I opened up the laptop and confirmed that the ribbon cable seemed to be connected at both ends. That was a bit worrisome. Could the disk be dead? Then came one of those good news/bad news situations. I booted the laptop from a live Mint USB drive. Once I was at the desktop, I was able to mount the hard disk and read it just fine. So the disk was not dead, which was the good news. The bad news was that this sent me off on a wild goose chase.

If I could read the hard drive when I booted from a USB, but I couldn't boot from it, logic said that the boot partition must either be corrupted or no longer marked as the boot partition. Right? A few episodes of cycling the power off and on confirmed that it consistently could read the hard disk when started from the USB stick and consistently could not boot from the hard disk. Only problem: running off the USB stick, I could read the boot partition just fine, and gparted confirmed that it was marked as the boot partition.

Unwilling to abandon my chase of the wild goose, I did the only thing I could think of: I reinstalled Mint (forcing me to update or reinstall tons of software -- still not done with that). After the reinstallation, the laptop booted from the hard disk just fine. I took it to the coffee shop and slugged down coffee while some of the software updates ran. Things were back on track ... until I got home and discovered it once again would not boot from the hard drive (same message as above). Grrrr.

Turns out a loose cable was at fault. It just didn't look (or feel) loose. In hindsight, the Broadcom ROM media test may just be more accurate (or more finicky) than whatever Mint does when deciding whether to mount a disk. For the benefit of anyone else staring at PXE-E61, here's a quick synopsis of what to check.

The first step is to remove the cover from the base of the laptop (Figure 1). (This might be a good time to apologize for the fuzzy images.)

laptop base
Figure 1: Laptop base

Remove one small (and easily lost!) Phillips-head screw from the base (highlighted in the photo), then slide the case in the direction indicated by the arrow and remove it, exposing the innards (Figure 2).

laptop interior
Figure 2: Laptop interior
The blue padding covers the hard disk; the arrow points to the ribbon cable connecting the hard disk to the (largely obscured) circuit board.

I'm used to cables that snap into place, but while this one connects solidly at the hard disk, the connection to the circuit board is secured largely by inertia. Figure 3 shows that end of the ribbon cable exposed. Note the metal "teeth"  (fingers? tentacles?). Also note the black line running across the width of the ribbon.

exposed cable end
Figure 3: Exposed cable end
Finally, Figure 4 shows the cable pushed all the way in at the circuit board end.

cable plugged in
Figure 4: Cable plugged in


You can see a light colored plastic tab covering the teeth of the cable, with a dark colored (and, it turns out, removable) plastic widget underneath the cable (visible on either side of the cable). Getting the cable to slide in between them is a bit of a pain in the ass. I don't know whether the dark plastic piece is a guide that was originally glued to the cable or not. In any case, the key is that the black line running across the width of the cable is close to the plastic stuff and, importantly, none of the metal teeth (fingers? tentacles?) are showing. If you see any portion of the teeth (connectors), the cable may be making intermittent content.

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