Encontrei isso aqui: link
Parece que pode ser o que você está experimentando.
After analyzing and trying things
out, I was able to solve this. It
seems that Boot Camp does not flag the
partition as ACTIVE immediately after
Boot Camp creates the new partition
from windows.
Assuming you already have a .dmg image
of your previous Windows Boot Camp
partition, do the following:
Use Boot Camp to allocate the new
space. Don't use Disk Utility because
the MBR will not be configured
properly
Use Disk Utility to restore your
.dmg image to the new Boot Camp
partition you just created. Be sure to
run IMAGES --> SCAN IMAGE FOR RESTORE
in Disk Utility before restoring. You
may need to unmount the Boot Camp
partition in Disk Utility before it
begins to restore.
-
Use fdisk in Terminal to mark the
Boot Camp partition active. First,
enter the fdisk MBR edit mode by
running the following:
$sudo fdisk -e /dev/disk0
Ignore the error "could not open MBR
file /usr/standalone/i386/boot0: No
such file or directory". Then,
determine which partition number to
mark active by running the following
(in bold):
fdisk: 1> show
Your windows partition should be #3
labeled "Win95 FAT-32". Now, mark the
partition active:
fdisk: 1> flag 3
Partition 3 marked active.
fdisk:*1> write
Device could not be accessed exclusively.
A reboot will be needed for changes to take effect. OK? [n] y
Writing MBR at offset 0.
fdisk: 1> exit
Now, reboot and hold the OPTION key
and Windows should be listed as a
bootable option.