Could not remove SCSI device

Hi,

Following is the procedure used to backup vmdk files of a VM using VDDK.
1. Take a snapshot of the VM.
2. Connect using VDDK
3. Open base vmdk file
4. Read vmdk files and do backup
5. Close vmdk file
6. Disconnect using VDDK
7. Delete the snapshot of the VM

We have tried backup of a VM using hotadd mode (using above procedure) and at the end(possibly during disconnect) it fails with following error in vddk output log file.

2010-12-29 23:09:12.193  diskLibPlugin: [2010-12-29 23:09:12.193 00552 verbose 'transport'] Could not remove SCSI device: adapter 0, target 1, error -536870373

2010-12-29 23:09:12.205  diskLibPlugin: [2010-12-29 23:09:12.205 00552 verbose 'transport'] Could not remove SCSI device: adapter 0, target 3, error -536870373

After this, the delete snapshot operation fails with error "Unable to access file since it is locked <unspecified file name>".

Please help us in finding what could cause to generate above error message.

Note: The backup works fine with hotadd mode.

Environment Details:
ESXi 4.1
VDDK 1.2
Guest OS: Windows 2008 R2

Thanks in Advance