When a backup is initiated, the third party backup provider creates a snapshot of the target VM. During a backup, opening a disk might take a longer time than the third party backup provider default timeout. When this happens, the third party backup provider considers the operation as a failure and as a consequence it tries to delete the snapshots. For example, a consolidation operation is initiated. The consolidation task will be started on the same disks which the third party backup provider attempted to open and caused the time out. Since the disks are still open, the consolidation will also fail, leaving stale delta files behind. The reason for the initial timeout and also why the disks are still in open state, is because when taking a backup of a VMDK file using SAN transport mode, a "Map Disk Region" call is initiated by VDDK to obtain the mapping of the disk. The use of SAN mode backups requires building up an entire mapping of the disk in-memory before allowing any I/O. This can take a very long time for large and/or fragmented disks.