[CentOS] mount: /dev/sdb1 already mounted or /blah busy

How do I go about troubeshooting this?  I'm using RHEL 4 update 6.

   mount: /dev/sdb1 already mounted or /blah busy

It's actually an iSCSI LUN (NetApp filer).  I successfully configured (ext3) and mounted it, but when I rebooted, the /dev/sdb1 device/partition is seen by the kernel and it shows up with "fdisk -l".

Nevertheless I get that error.  I've tried "umount" and am informed that /blah isn't mounted.  I've tried "lsof" and don't find a process that's attached to /blah.

I also tried "fsck -t ext3 /dev/sdb1" which worked (so it detects the ext3 file system).

Any other tools to check out?
   === Al