Drive letter changes when booting with external monitor

itsdrike · n00b Joined: 19 Jul 2021 Posts: 4

So, this is pretty weird, basically until now I've been using the distribution kernel, which worked well, but I wanted to try and use my own kernel. I managed to set everything up, however I'm having a really weird issue:

I'm using grub as my boot manager, and it has this line in it linux /boot/vmlinuz-5.13.1-gentoo root=/dev/sda4 ro, I'm not sure why it isn't using UUIDs, but that's not really the issue here, this works fine and I can boot using it without any problems. However for some really weird reason, if I boot while having an external HDMI monitor plugged to my laptop, it goes to kernel-panic, complaining that there's no such drive and showing me a list of partitions, that are now under /dev/sdb instead of /dev/sda. When I change the grub config to use that partition instead, it works and gets me into the system without issues.

I see no real reason why plugging and HDMI display should in any way change the drive letter to /dev/sdb, when I booted like this and checked fdisk -l, there was no other drive detected which would take the up place, it simply only listed my main single drive as /dev/sdb. This issue does not happen with the distribution kernel, which means it must be an issue with my kernel configuration, but I can't seem to be able to figure out which setting could be causing something like this.

Any suggestions?

eccerr0r · Posted: Mon Jul 19, 2021 10:32 pm Post subject:

You should be using UUID to get around this problem. It's an option in /etc/default/grub GRUB_DISABLE_LINUX_UUID - hopefully that's commented out or set to no. Also it may be affected by whether or not you use an initramfs like from dracut or genkernel... at least in the past UUID required initramfs to parse.

Device number changing is actually quite possible with docking stations and how fast each (USB) device takes to respond when things are connected or not. I'd imagine PCI/PCIe/mPCIe enumerate fast enough whether or not devices are connected or not that it shouldn't matter.
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itsdrike · n00b Joined: 19 Jul 2021 Posts: 4

itsdrike · n00b Joined: 19 Jul 2021 Posts: 4

eccerr0r · Posted: Mon Jul 19, 2021 11:21 pm Post subject:

The other possibility is that the other distributions have drivers in the initramfs and yours are hardcoded. The hardcoded ones are pulling in drivers for your expansion box and video in a different order, shifting enumeration a bit...

TBH I don't even notice sdX devices unless working with physical devices... root filesystem is agnostic to the device it's installed on!
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Intel Core i7 2700K/Radeon R7 250/24GB DDR3/256GB SSD
What am I supposed watching?

itsdrike · n00b Joined: 19 Jul 2021 Posts: 4

eccerr0r · Posted: Mon Jul 19, 2021 11:27 pm Post subject:

I think there is through udev rules (though not sure how well it works on root devices), but I never researched it, was never a problem for me.
That is, other than leaving all drivers out of the kernel and putting unneeded-for-boot driver in initramfs.
_________________
Intel Core i7 2700K/Radeon R7 250/24GB DDR3/256GB SSD
What am I supposed watching?

Goverp · Advocate Joined: 07 Mar 2007 Posts: 2006

While most initramfs scripts can handle root=UUID=foo, without one, the kernel handles root=PARTUUID=bah, but not UUID=foo. Use blkid to get it.
The comments in the source code give full details.
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Greybeard