Okay here is the run on the other partition, including a pre-conversion 
fsck.

# df -h /raid1
Filesystem            Size  Used Avail Use% Mounted on
/dev/sdb1             1.8T  1.2T  557G  69% /raid1

*Pre-conversion fsck:*
# time fsck -fp /dev/sdb1
fsck 1.41.3 (12-Oct-2008)

RAID1: 3259/244203520 files (13.8% non-contiguous), 318202066/488382016 
blocks

real    70m24.617s
user    16m50.522s
sys    4m47.142s

# tune2fs -O extents,uninit_bg,dir_index /dev/sdb1
tune2fs 1.41.3 (12-Oct-2008)
Please run e2fsck on the filesystem.

*The painful fsck that does all the work:*
# time fsck -fp /dev/sdb1
fsck 1.41.3 (12-Oct-2008)
RAID1: Group descriptor 14902 checksum is invalid.  FIXED.
RAID1: Group descriptor 14903 checksum is invalid.  FIXED.
RAID1: Group descriptor 14904 checksum is invalid.  FIXED.

RAID1: 3259/244203520 files (13.8% non-contiguous), 318202066/488382016 
blocks

real    70m3.541s
user    16m27.907s
sys    4m44.266s

*Post-conversion fsck:*

# time fsck -fp /dev/sdb1
fsck 1.41.3 (12-Oct-2008)
RAID1: 3259/244203520 files (13.8% non-contiguous), 318202066/488382016 
blocks

real    43m50.323s
user    1m51.994s
sys    0m37.605s


-Charles