not a valid block device on a partition that formerly mounted

[Margaret Doll]

I have a LaCie BigDisk Extreme attached through USB on my RedHat system.

This disk has never mounted on boot up. I had to wait until the boot  
up was complete, complete /etc/fstab with mount information for the  
partitions on the disk and then mount the partitions.

Sometime during the last week, the partitions became unmounted and I  
can no longer get them mounted.

The system sees the device on reboot. From dmesg

Vendor: LaCie Model: BigDisk Extreme Rev:
Type: Direct-Access ANSI SCSI revisiuon: 02
Attached scsi disk sdb at scsi2, channel 0, id 0, lun 0
SCSI device sdb: ...
sdb:<6>kjournald starting. Commit interval 5 seconds

The device should have five mountable partitions.

"fdisk -l" only shows partitions for /dev/sda
"fdisk /dev/sdb" returns "Unable to open /dev/sdb"

I am running RedHat 2.4.21-53.ELsmp on the system.

How do I get the partitions on the disk to mount again?