Backup - Unable to create volume snapshot (0x70021)

Trying to backup the drive to a USB memory stick. The stick has more than enough space to backup the drive. Memory stick format NTFS.
Windows 7 64x computer.
Backup proceeds as normal then ends with no backup on the memory stick.
Next to last status message:
Operation with partition 0-0 was terminated. Details: Unable to create volume snapshot (0x70021)
Tag = 0xA5695862AAF8E645 Failed to lock volume snapshot (0x10D449)
Tag = 0x14181C22EFA36 Already locked (0x9)

Anyone else seen this issue.
Using TrueImage OEM 2010 build 1,542

Thanks