Summary and Info:
Arch 4.10.3-1
mkinitcpio version 23

Steps to Reproduce:
Disconnect power when Arch is running of a USB stick with 3 partitions:
1. EFI boot vfat
2. root ext4
3. home ext4

Hoping that this will trigger setting the dirty bit.

Even after manually adding the BINARY="mkfs.vfat" to mkinitcpio.conf the computer won't automatically repair the FAT errors. Instead the boot process hangs at:

mount: unknown filesystem type 'vfat'
Your are now being dropped into an emergency shell.


When boot from livecd and chroot into, these errors are shown:
# fsck.vfat /dev/sdb1
fsck.fat 4.1 (2017-01-24)
0x41: Dirty bit is set. Fs was not properly unmounted and some data may be corrupt.
1) Remove dirty bit
2) No action
? 2
There are differences between boot sector and its backup.
This is mostly harmless. Differences: (offset:original/backup)
65:01/00
1) Copy original to backup
2) Copy backup to original
3) No action
? 3
/dev/sdb1: 17 files, 10125/261628 clusters

A liveCD chrooted manual command "mkfs.vfat -a /dev/sdb1" wipes all files from the EFI partition.