This Citrix hotfix corrects one of the root causes for snapshot errors. Links to release notes and downloads can be found here.
Citrix has released Hotfix XS61E034 - For XenServer 6.1.0 which addresses corruption issues that have been seen when using VDIs 2048 GB in size. For more information and to download the hot fix please refer to this Citrix article:
If you are running XenServer 6.1 with a storage repositories (SR) greater than 1 TB you may experience issues deleting snapshots. Citrix has released Hotfix XS61E015 - For XenServer 6.1.0 to correct the issue. Information about downloading and applying the Hotfix can be found here: http://support.citrix.com/article/CTX137996
Virtual Backup Appliances call the Xen API to create and delete snapshots during the backup process. This hotfix may address one of the root causes of the SR_BACKEND_FAILURE_44 or SR_BACKEND_FAILURE_109 errors that may have been encountered when running backups.