My experience is that when doing an incremental backup, that SuperDuper copies files across to the backup first, before deleting the outdated files.
So it is possible that if you don't have much HD space on the backup volume, which should be similar to the source), then SD can't copy the updated large image file onto the backup volume to start with. The solution was to delete a selection of files from the backup volume manually, right before performing the incremental backup.
This happened to me quite a bit, and wasn't specific to Parallels. Although Parallels may aggravate the problem since the program -- by design -- works with large image files.
So it is possible that if you don't have much HD space on the backup volume, which should be similar to the source), then SD can't copy the updated large image file onto the backup volume to start with. The solution was to delete a selection of files from the backup volume manually, right before performing the incremental backup.
This happened to me quite a bit, and wasn't specific to Parallels. Although Parallels may aggravate the problem since the program -- by design -- works with large image files.