I have a 4 year old Unraid server with Pro key installed. 

This morning feeling I need something to do, don't ask me why, I took the initiative to upgrade when I noticed that my Unraid version is about 12-18 months. This is normal as I don't want to upgrade too soon. 

After the upgrade, it won't boot, something about the USB drive. Stupidly, I then downloaded 6.12.3 on my Windows PC and use the USB creator to put that on the SAME USB flash. Now I managed to boot up to the login screen, but it's asking for changing my boot password and it's asking for my Unraid key, I also noticed while it listed all my drives, but everything has to be put back in the same order. I realized I probably don't have any backup of the settings somewhere, even worse the settings are probably in those drives. What recourse do I have now? I'm afraid to proceed further perhaps risking do damage all the files sitting in unassigned drives. I may have pics of the drives setup and potentially can put them back in the right order. But what can I do?

Thanks.

You can use this:

https://docs.unraid.net/unraid-os/manual/changing-the-flash-device#what-to-do-if-you-have-no-backup-and-do-not-know-your-disk-assignments

I confirm that I have screenshot of the server arrays including the discs' serial no, so theoretically I can put them in the correct order and boot up. But I have previously set encryption, I have to re-enter a key when the array is brought up, is this going to be an issue? I also have various apps and dockers, will those have to be manually re-done? I felt like I bite more than I can chew, mostly because my previous upgrade has not resulted in something like this that I "forgot" the risk of upgrade. 

Thanks JorgeB for the link. There's a warning in red that "All existing data on this device will be OVERWRITTEN when array is Started", how true is this? Assuming I have the arrays in the correct order? What if only one disk is not assigned correctly?

That's only for the drive(s) assigned to parity, and if you know the previous assignments you can check "parity is already valid" before array start to avoid a re-sync.

Thanks again JorgeB! Happy ending for what was quite a scare. 

I was looking for screenshot of the array setup when I came across the USB backup from 9 months ago, suffice to say, it's "recent enough" to get my server back up and running again. Without this, I'd have to restart the array following the above link that JorgeB provided. 

I noticed my Unraid is now 6.10.3, but I thought I was upgrading to 6.12.3? Maybe that question will be for another topic. 

Adding to the scare, Unraid website was undergoing maintenance when I was having this issue, so if I were to download another version of the OS, I'd have to wait a couple more hours. What a coincidence! Such is life.