mjhjmtu

(I no longer have this setup so I cannot test new answers, sorry. I had to switch computers and am now only using arch; I am leaving the question as it is though since I'd love to know a solution if it occurs again, grub has been very hostile towards me these past few months)
I have fedora 23 as my main OS with custom partitioning (all physical partitions) /boot/efi=/dev/sda1 root=/dev/sda3. Then on /dev/Sda2 I just set up an LVM physical partition, and created an LVM group (arch) and volume (root) and then I installed Arch Linux (without bootloader) on /dev/arch/root

I have /dev/sda (GPT), /dev/sdb (GPT) then grub also seems to detect a hd2 and gives errors about being unable to load it (I assume this is the LVM physical partition) anyhow, I use os-prober and grub2-mkconfig to detect my arch installation, which it successfully does (and does so two times, I get two menu entries for it); but when I try to boot it I get the following errors:

error: failure reading sector 0x0fc from 'hd2'.
error: failure reading sector 0x0e0 from 'hd2'.
error: failure reading sector 0x0 from 'hd2'.
error: can't find command 'linux'.
error: can't find command 'initrd'.

I only get the sector reading errors on the first boot attempt (for the record, fsck reports the drive to be clean) any retries only give me the can't find command errors.
This is the grub.conf menuentry:

### BEGIN /etc/grub.d/30_os-prober ###
menuentry 'Arch (on /dev/mapper/arch-Root)' --class gnu-linux --class gnu --class os $menuentry_id_option 'osprober-gnulinux-simple-38305dfb-279b-4490-948b-480b81fef81f' 
 insmod part_gpt
 insmod lvm
 insmod ext2
 set root='lvmid/6ZmQFy-ijXr-mYra-3Gp9-l0dh-J4Wi-GSHXhd/WGN2VN-t34t-rYWi-kvje-2BfF-WoD4-4NinnP'
 if [ x$feature_platform_search_hint = xy ]; then
 search --no-floppy --fs-uuid --set=root --hint='lvmid/6ZmQFy-ijXr-mYra-3Gp9-l0dh-J4Wi-GSHXhd/WGN2VN-t34t-rYWi-kvje-2BfF-WoD4-4NinnP' 38305dfb-279b-4490-948b-480b81fef81f
 else
 search --no-floppy --fs-uuid --set=root 38305dfb-279b-4490-948b-480b81fef81f
 fi
 linux /boot/vmlinuz-linux root=UUID=38305dfb-279b-4490-948b-480b81fef81f rw quiet
 initrd /boot/initramfs-linux.img

Anyone got any ideas how to fix this? Any solution that will allow me to boot into arch (besides reinstalling the bootloader from arch instead of fedora) will be fine; I don't mind getting my hands dirty editing the grub.cfg file by hand.

I had installed Debian without LVM partitioning and next to that I installed Fedora which installed with LVM when using the guided partitioning on the remaining disk space. Since debian installed without LVM it couldn't read the fedora partitions to add a grub entry for it when running grub2-mkconfig. What worked for me was installing the lvm2 package on Debian and running update-grub.

You said you installed Arch without a bootloader. The errors you posted hint that there is no valid boot sector at all. IIWY I'd install Arch Linux, then install Fedora 23. The fedors installer will overwrite the boot sector with it's grub, and worst case, you can use a custom grub entry to chainload Arch.

I'm pretty far from being a grub expert, but I did just get my laptop booting Windows 7, Solaris 11, CentOS 7, and Ubuntu 14.04 :-D I had to do some tapdancing!