Install from: USB flash drive
Install mode: x64
Install options: mdraid per partition (or not)

As of ISO Nov 2014, if too many packages are installed by pacstrap, pacstrap crashes shortly after mkinitcpio runs. The last pacstrap commands and user typed commands show these errors.

(211/211) installing ifplugd [######################] 100%
/usr/bin/pacstrap: line 281: /usr/bin/cp: Input/output error
/usr/bin/pacstrap: line 287: /usr/bin/cp: Input/output error
/usr/bin/pacstrap: line 106: /usr/bin/umount: Input/output error
126 root@archiso ~ # umount -R /mnt/install :(
zsh: Input/output error: umount

The console periodically squirts tens of errors like these.

device-mapper: snapshots: Invalidating snapshot: Unable to allocate exception.
Buffer I/O error on device dm-0, logical block ....
EXT4-fs error (device md-0): ext4_read_inode_bitmap: comm zsh: Cannot read inode bitmap block_group = ..., inode_bitmap = ...
EXT4-fs (dm-0): previous I/O error to superblock detected

At this point the system is non functional. The kernel is in a soft panic and mostly non functional. The zsh shell prompt is up and accepts commands. A few commands work (uname, cat, vi, mdadm, dd, swapon, blockdev). Most commands fail with an Input/Output error (tail, desmg, umount, mc, blkid, ls). systemctl reboot won't work. A message is displayed but the reboot doesn't happen. Press Ctrl+Alt+Del "Nothing happens..." After reading a few messages on the screen I must power off.

The error never occurred in about 10 install attempts from ArchISO 2014.10 to 2014.05 in about 10 install attempts.
The error occurred in every of 30 attempts from ArchISO 2014.11 to 2015.05.
The same error occurs whether MBR or GPT.
The same error occurs whether boot from a USB 2.0 port or a USB 3.0 port.
The same error occurs whether system RAM is 8GB or 32GB.
The same error occurs whether from a Kingston DT USB 2.0 produced with rawrite or a PNY USB 3.0 produced with YUMI.
The same error occurs whether installed to the SAS drives or the SATA drives. The SAS and SATA drives are not in the system at the same time.
The same error occurs whether all 4 SAS drives are present or 3 of them are ejected.
The same error occurs if the two big partitions are formatted ext3 instead of ext4. The errors still say EXT4 and not ext3. I also tried xfs which doesn't format the large /home partition right so it mounts readonly. The errors continue to say EXT4-fs despite the only ext partition being /boot as ext2.
The same error occurs with or without mdraid.
The errors always start after pacstrap runs mkinitcpio. The only kernel version tested was 3.19.2-1-ARCH. The previous Live ISO pacstrap that works is installing the exact same version.
The same error occurs if I create a single ext4 partition and no swapfile.
The same error occurs if I install on a Haswell C226 platform or a Intel i945 platform.

To get around this error I had to split up my pacstrap into the following 3 lines.

pacstrap -c /mnt/install/ base syslinux cronie mdadm mc vim

Later in chroot: pacman -S base-devel openssh sudo ifplugd gdisk

Later after first boot: pacman -Sy hdparm zsh nfs-utils nmap autofs sshfs autossh gnu-netcat samba ntp iftop htop lsof strace tcpdump wol lynx rsync git pkgfile wget lftp dos2unix cpio cabextract cdrkit aspell mtools unace unarj unrar zip unzip p7zip

To reproduce the crash simply pacstrap all of the above packages at the same time.

I was unable to narrow the crash to a single package (note the 30+ installs above). The actual packages seem to not make a difference. The excessive number of packages seems to be what causes the crash.