Follow the instructions here to install the fastboot driver for Windows: https://grapheneos.org/install/web#connecting-device
Undefined error on google chrome web installer
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I have the same with the web install method. After trying to download release with its blue button, I get in red: Error: undefined. Is repeatable, but stops at different downloaded bytes.
Also, when trying the cli method it stops after trying to flash the first slot, then the phone doesn't react on any further fastboot command except fastboot devices. Knobs are pretty much stuck at this point, the only key combination to save one's butt is holding Rescue and vol up at the same time for about 10 seconds or so, then it boots into stock Android but with an unlocked bootloader...
Could flash it back with stock Android yesterday, now there's a new one, still trying after failing with GrapheneOS again on this device... (bluejay)
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de0u That sounds like it might be a USB cable/port problem. What kind of cable are you using, and where is it plugged in?
The original cable (USB-C to USB-A) which came with the device, and I tried two different USB-A ports on the front of my machine (the back is hard to reach, that's where keyboard, mouse, and audio interface are used). Normal data transfer or charging works nicely with those front ports (I've built the machine myself, should be good).
OS: tried with both Arch and Debian Bookworm, could also try with Windows if that had to be, but I don't know much about Windows anymore...
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treequell Use the USB ports on the back of your machine. The ones on the front usually aren't suitable for flashing GrapheneOS.
Hm, I don't know if I have any free in the back, can look for that tomorrow. The front ones are 2 x USB 3.2 Gen 1 Ports, the back ones 4 x USB 3.2 Gen 2 ports so yes these are a bit speedier. MB is https://rog.asus.com/motherboards/rog-strix/rog-strix-b550-i-gaming-model/ in a mini-ITX case (Cooler Master NR200)
Edit: one of the the front ports is currently charging the Pixel 3a, while the 6a is on the wall socket charger, updating stock Android while I type... bugs me that the boot loader is still open...
2nd edit: just checked, started into fastboot again, and then it's like this:
wjl@handa-6:~$ fastboot devices
29011JEGR15311 fastboot
wjl@handa-6:~$ fastboot flashing get_unlock_ability
< waiting for any device >
^C
wjl@handa-6:~$
And no matter which fastboot command from the .sh script I try manually, it's always that answer - guess that is the reason that the web installers (both Graphene's and also Google's) also get stuck at this point... tried factory reset yesterday, didn't help. No idea at which point the Android web installer then worked, and why (I was still on platform tools 33 on Debian yesterday, now it's 34.0.4)... anyway, it's after midnight here, so thanks for now and good night...
wjl Hm, I don't know if I have any free in the back, can look for that tomorrow. The front ones are 2 x USB 3.2 Gen 1 Ports, the back ones 4 x USB 3.2 Gen 2 ports so yes these are a bit speedier.
The reason why the installation directions say to use the direct-to-motherboard ports instead of the extended ports is because the extended ports result in flashing failures much more often (even though they work ok for charging and for low-speed/occasional data transfer). So it's genuinely worthwhile to try this step (and all the steps) exactly the way that the directions indicate.
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wjl Please follow the installation guide on the GrapheneOS website. It explains not to do exactly what you're doing i.e. using the USB ports on the front of your machine...
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I have only a USB Type C port free on the back of my machine; however I can still try to temporarily unplug my monitor's USB cable from one of the USB A ports.
That doesn't however solve the problem with the unsuccessful downloads from the web installer. Still repeatedly getting:

no matter if I try from Debian or Arch, no matter if I try Chromium or the original Google Chrome browser without any extensions...
To my limited knowledge that should have nothing to do with any USB cabling and/or port, or does it?
Anyway, thanks all...
wjl I recommend that you use the USB-C port on the back of your device, and the factory USB-C to USB-C cable from Google, if you have it.
wjl ](https://)
This error is most likely caused by your USB connection dropping.
Make sure also that you have at least 32GB free storage space on your PC.
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treequell I just did, temporarily disconnected my audio interface to do so. No difference.
treequell This error is most likely caused by your USB connection dropping.
I thought so too in the beginning - but as it can be repeated with GrapheneOS (haven't tried others on this 6a) I think somehow it corrupts the first phone slot after flashing it, and then stops. I've tried that twice now, the rescue mode then booted stock Android from slot B which saved me.
treequell Make sure also that you have at least 32GB free storage space on your PC.
On my /home I have 32.5GiB/256GiB, on my /bigdisk (still rotating platters) it's 1.99TiB/3.58TiB. That's the occupied/total bits...
I could flash it back to stock with Google's web installer, and that option also locked my bootloader again - now it's on the August patch of Android 13, which I'll keep for now.
But thanks for the help everyone. I still think GrapheneOS is golden, although on the 3a I now changed to DivestOS (easily and without any problems installed from my front USB btw) as suggested. That's a good option for devices not covered here anymore indeed, so thanks for that tip as well.
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wjl easily and without any problems installed from my front USB btw
Okay? I've had zero issues installing GrapheneOS on numerous Pixels devices over the years.
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wjl I thought so too in the beginning - but as it can be repeated with GrapheneOS (haven't tried others on this 6a) I think somehow it corrupts the first phone slot after flashing it, and then stops. I've tried that twice now, the rescue mode then booted stock Android from slot B which saved me.
That's incorrect. The GrapheneOS web installer is not doing that. There are no such issues with web installer. The installation is failing because you have not configured something correctly.
The process used to flash other OSes is not the same as flashing GrapheneOS, so being able to flash one OS is not indicative either way.
If you're still struggling with your PC, I suggest that you use another device to flash GrapheneOS. I would recommend running the web installer in Chrome on a device running Android or MacOS. Your Pixel 3a phone might work as an installer.
[deleted] I've had zero issues
That may well be. I also had zero issues installing all kinds of third party operating systems on numerous Nexus and Pixel devices over the years, and GrapheneOS installed nicely (via cli) on the Pixel 3a. So I guess it must me my bluejay device which behaves somehow strange.
treequell There are no such issues with web installer.
Of course not - that was what I saw using the cli installer, the flash-all.sh script stopped after flashing the first slot, so from there on (after rebooting the device) I tried those lines one by one myself, just with copy & paste from the script to the command line.
treequell If you're still struggling with your PC
I don't think that I'm stuggling with my PC. I'm retired by now, but before that I helped the Debian project a bit, was a system administrator in healthcare IT, and also with IBM for several years in technical support. I think I know my way around computers, except that I don't like Windows or Mac much (have been on Linux for over 20 years, also tried some BSDs and supported Solaris before that).
But no bad feelings, again I thank you all, and it's all good. For the moment I'll stay on stock Google Android (as do my wife and daughter anyway), and on the 3a Graphene was awesome, but DivestOS is also nice.
Thanks all.