I opened up my PC case and took out two unused drives and added in two new drives which I intend to use as (mirrored) cache. Unraid would not boot, so I took out the two new SDDs. At this point my system should be identical to what it was before, with the exception of the removal of those two unused drives. Unraid will no longer boot. I've tried with and without the GUI and with and without Safe Mode. It gets as far as "sd 7:0:0:0: [sdd] Preferred minimum I/O size 4096 bytes" and then appears to just hang. I really don't know how to debug this. I made a bootable Ubuntu USB and booted that up; the hardware all appears to be fine. Ubuntu found my two 4TB Array drives and my two 256MB Cache drives.

Any suggestions for next steps would be most welcome.

As a first step, try recreating the flash drive, first backup the current one, then recreate it using the USB tool, confirm the flash drive boots with the stock install, and if yes, restore only the /config folder from the backup overwriting any existing files.

9 hours ago, JorgeB said:

As a first step, try recreating the flash drive, first backup the current one, then recreate it using the USB tool, confirm the flash drive boots with the stock install, and if yes, restore only the /config folder from the backup overwriting any existing files.

Interesting idea, Jorge. What I learned first of all is that the Unraid USB Creator doesn't make the USB drive bootable. <grrr> After wasting an hour or more trying four different USB drives and three different ports on my PC, I finally stumbled upon "make_bootable.bat". Now that I'm past that hurdle, I see that my original USB will boot just fine with the new config folder, but with my saved config folder the boot continues to hang at the exact same spot. I guess that means there is some setting in there that's completely tripping up Unraid. It also means that as a last resort I can rebuild Tower from scratch, since my data are all on the rotating disk. But, of course, I'd really like to just get my original config working.

You should be able to examine most of your config in a text editor.

super.dat (array assignments) is not simple text, and many plugins will have .txz files in their folders, but the .cfg files at the top of config on the flash drive should be simple text. See if any look corrupt.

13 hours ago, nkn56 said:

What I learned first of all is that the Unraid USB Creator doesn't make the USB drive bootable.

It does make it bootable via UEFI, if you are booting legacy, you need to run make_bootable. It does warn that once it's finished writing the flash drive.

Regarding the issue, you can try to just restore the bare minimum from the backup config, like the key, super.dat and the pools folder for the assignments, also copy the docker user templates folder (\config\plugins\dockerMan\templates-user), if all works you can then reconfigure the server or try restoring a few config files at a time from the backup to see if you can find the culprit.

It could also be worth booting in Safe Mode with the original 'config' contents to see if the problem is a plugin rather than one of the standard Unraid configuration files.