Windows Backup fails to create Shadow Copy

Windows Backup is failing with:

A shadow copy could not be created.
Please check "VSS" and "SPP"
application event logs for more
information. Details: The writer
experienced a non-transient error. If
the backup process is retried, the
error is likely to reoccur.

The VSS event log says:

Volume Shadow Copy Service error:
Unexpected error calling routine
IVssAsrWriterBackup::GetVolumeComponents.
hr = 0x80073bc3, The requested system
device cannot be found. .

Operation: OnIdentify event
Gathering Writer Data

Context: Execution Context: ASR
Writer Writer Class Id:
{be000cbe-11fe-4426-9c58-531aa6355fc4}
Writer Name: ASR Writer Writer
Instance ID:
{108b2193-aab4-4e86-a66a-5232a580ee7e}

The SPP log says:

Shadow copy creation failed because of
error reported by ASR Writer. More
info: The requested system device
cannot be found. (0x80073BC3).

System is 64-bit Windows 7. I'm trying to back up one internal drive to another (C->E). C: is a 111GB partition on a 160 GB Intel SSD (The System Reserved partition is 100MB). E: is a 2TB Hitachi hard drive (one partition, NTFS formatted).

Update:

Perusing the links provided by echarnley gave me the idea to try marking the C: partition active (it wasn't). That let me backup an image of C: without error, but if I selected any locations for files to backup, it failed with:

The system cannot find the file
specified. (0x80070002)

Marking the system reserved partition active instead solved that problem, full backups (image + files) now complete without error.