[ubuntu] HDIO_GET_IDENTITY failed for '/dev/sdb' after upgrade

Three weeks ago I upgraded my dual boot (Windows/Ubuntu) Asus computer to ubuntu 13.04. The configuration was different - in that it designated the Windows partition as OS - but it worked. Until today.

I booted and got this error message:
ata_id (296:) HDIO_GET_IDENTITY failed for /dev/sdb: Invalid argument

Then a message BusyBox v1.20.2 etc

and a prompt:
(initramfs)

I've searched to find how others have dealt with this error but mainly find people using external drives ...

All help appreciated.
Thanks.

Howard (htubi)