Mdadm RAID 5 array stopped mounting

I added a 2TB drive to my system (NOT to my RAID 5 array, just an additional single drive), and now the array won't mount – I get a "mount: /dev/md0 already mounted or /media/storage busy" error. Everything's worked fine for 2 years. All I did was successfully update the MB bios, attach the 2TB Samsung hard drive, formatted it ext4 with a 1MB offset using a gparted bootable CD, then restarted my regular Ubuntu 9.10 system, at which point the array doesn't mount and I get the "mount: /dev/md0 already mounted or /media/storage busy" error.

mdadm says everything's still fine:

mdadm –detail /dev/md0
/dev/md0:
Version : 00.90
Creation Time : Sat Aug 2 16:39:27 2008
Raid Level : raid5
Array Size : 2197715712 (2095.91 GiB 2250.46 GB)
Used Dev Size : 732571904 (698.64 GiB 750.15 GB)
Raid Devices : 4
Total Devices : 4
Preferred Minor : 0
Persistence : Superblock is persistent

    Update Time : Sat Nov  6 08:17:54 2010
          State : clean
 Active Devices : 4
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 64K

           UUID : 59d14b52:c05d4166:1246e5fe:589d0142
         Events : 0.3735628

    Number   Major   Minor   RaidDevice State
       0       8       17        0      active sync   /dev/sdb1
       1       8       33        1      active sync   /dev/sdc1
       2       8       49        2      active sync   /dev/sdd1
       3       8        1        3      active sync   /dev/sda1

scan looks fine too:

mdadm –detail –scan
ARRAY /dev/md0 level=raid5 num-devices=4 metadata=00.90 UUID=59d14b52:c05d4166:1246e5fe:589d0142

But I can't mount it:

mount /media/storage
mount: /dev/md0 already mounted or /media/storage busy

mdadm.conf still seems correct and matches scan output (and worked fine for 2 years until now):

# mdadm.conf

# by default, scan all partitions (/proc/partitions) for MD superblocks.
# alternatively, specify devices to scan, using wildcards if desired.
DEVICE /dev/sd*

# auto-create devices with Debian standard permissions
CREATE owner=root group=disk mode=0660 auto=yes

# automatically tag new arrays as belonging to the local system
HOMEHOST <system>

# definitions of existing MD arrays
ARRAY /dev/md0 level=raid5 num-devices=4 UUID=59d14b52:c05d4166:1246e5fe:589d0142

…and fstab looks correct (and has worked for 2 years as well):

# /etc/fstab: static file system information.

# <file system> <mount point>   <type>  <options>       <dump>  <pass>
proc            /proc           proc    defaults        0       0

# /
UUID=4204d07b-c450-4461-a001-1ef6d4ecb1a3 / ext4 errors=remount-ro 0 1

# swap 
UUID=95331961-9abd-4795-9443-292146aca413 none swap sw 0 0

# RAID storage drive
/dev/md0 /media/storage ext3 auto,rw,user,relatime,noatime,nodiratime,exec,async 0 3

I'm at a loss for what to try next…