Can't boot after update, getting "microcode: failed to load" error

Question

After restarting my computer, it's no longer able to boot, and just prints a continuous stream of errors. The first error it gives is "microcode: failed to load", and I looked it up and found that I should just have to install amd-ucode. Easy enough, except I can't boot. I tried booting into the latest live-usb image, and after connecting to the internet, mounting the partition to /mnt, and running
# arch-chroot /mnt pacman -S amd-ucode
I get the following error
error: amd-ucode: signature from "Allan McRae <[email protected]>" is invalid
error: failed to commit transaction (invalid or corrupted package (PGP signature))
Errors occurred, no packages were upgraded.
I'm not very familiar with how chrooting works, especially not arch-chroot, so I don't trust myself to fix the problem without breaking something. I was thinking of just putting the package on a flashdrive and installing it with
# arch-chroot /mnt pacman -U amd-ucode-package-name
Should I do it this way or am I misunderstanding how arch-chroot works?
By the way, this is an issue with my desktop. startx stopped working on my laptop right before I went to use my desktop and rebooted it, so after this issue is fixed I'll be making another thread about my laptop >_> Any help is much appreciated.

Answer 1

Ok, so I managed to install amd-ucode with pacstrap. That got rid of the microcode error, but I'm still getting
/dev/sdc1 contains a filesystem with errors. Forcing check
And then it just repeats the error message
[60.50342] ata5.00: failed command: READ FPDMA QUEUED
[60.51345] ata5.00: cmd 60/<long number that's formatted similarly to a MAC address>/40 tag 0 ncq 131072 in
[60.51743] res 40/<the same number>/40 Emask 0x10 (ATA bus error)
[60.62421] ata5.00: status: { DRDY }
The numbers on the left just keep counting up, but the repeating error message stays mostly the same. What's odd about this error is that the disc it says it has a problem with isn't even the disc with the OS on it. When I have time I'm going to try physically removing /dev/sdc from my computer to see if the error stops.
Oh and there won't be any thread about my laptop issues. Turns out I filled my whole HDD, which was causing startx to fail.