panic : swap_pager

[Zhang hw]

Sometimes when I shutdown my system, it would tell me :
"...
 All buffers synced
 Swap_pager: I/O error - pagein failed, blkno 744, size 4096, error 5
 panic: swap_pager_force_pagein: read from swap failed
 Uptime:...
 Cannot dump. No dump device defined.
 Automatic reboot in 15 seconds -> press a key on the console to abort"
If press a key, the system will show:
"->Press a key on the console to reboot
 ->or switch off the system now"
uname -a: 6.2-RELEASE-p6 FreeBSD 6.2-RELEASE-P6 #3
Sometimes the blkno mybe with a different number after it.
Who can tell me how could deal with it?
Thanks!