Attempting to unmount /backup drive crashes server.

Anyone ever heard of this one:

When the cpbackup routines attempts to umount the backup drive, the server goes into some kind of CPU runaway and then crashes:
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top - 01:16:44 up 23:50, 1 user, load average: 83.40, 57.89, 28.13
Tasks: 221 total, 7 running, 214 sleeping, 0 stopped, 0 zombie
Cpu0 : 0.0%us, 0.0%sy, 0.0%ni, 0.0%id,100.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu1 : 0.0%us,100.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu2 : 0.0%us, 0.5%sy, 0.0%ni, 0.0%id, 99.5%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu3 : 0.0%us, 0.0%sy, 0.0%ni, 0.0%id,100.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 2074148k total, 1962620k used, 111528k free, 198340k buffers
Swap: 2040212k total, 0k used, 2040212k free, 990440k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
2268 root 36 19 2192 644 536 R 100 0.0 12:57.58 umount /backup

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mtab looks like this:

/dev/sda5 / ext3 rw,usrquota 0 0
proc /proc proc rw 0 0
sysfs /sys sysfs rw 0 0
devpts /dev/pts devpts rw,gid=5,mode=620 0 0
/dev/sdb1 /backup ext3 rw 0 0
/dev/sda1 /boot ext3 rw 0 0
tmpfs /dev/shm tmpfs rw,noexec,nosuid 0 0
/dev/sda8 /home ext3 rw,usrquota 0 0
/dev/sda6 /tmp ext3 rw,noexec,nosuid 0 0
/dev/sda3 /usr ext3 rw,usrquota 0 0
/dev/sda2 /var ext3 rw,usrquota 0 0
none /proc/sys/fs/binfmt_misc binfmt_misc rw 0 0
/tmp /tmp none rw,noexec,nosuid,bind 0 0


fstab looks like this:

LABEL=/ / ext3 defaults,usrquota 1 1
LABEL=/backup /backup ext3 defaults 1 2
LABEL=/boot1 /boot ext3 defaults 1 2
devpts /dev/pts devpts gid=5,mode=620 0 0
tmpfs /dev/shm tmpfs defaults,noexec,nosuid 0 0
LABEL=/home /home ext3 defaults,usrquota 1 2
proc /proc proc defaults 0 0
sysfs /sys sysfs defaults 0 0
LABEL=/tmp /tmp ext3 defaults,noexec,nosuid 1 2
LABEL=/usr /usr ext3 defaults,usrquota 1 2
LABEL=/var /var ext3 defaults,usrquota 1 2
LABEL=SWAP-sda7 swap swap defaults 0 0


And here's the df result:

Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sda5 8123168 393920 7309956 6% /
/dev/sdb1 378473076 9409988 349527648 3% /backup
/dev/sda1 124427 15256 102747 13% /boot
tmpfs 1037072 0 1037072 0% /dev/shm
/dev/sda8 206970004 16014836 180271948 9% /home
/dev/sda6 1019208 38932 927668 5% /tmp
/dev/sda3 8123200 3612156 4091748 47% /usr
/dev/sda2 10154020 734380 8895520 8% /var



Does anyone see anything wrong here?

Thanks very much for anything.