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Quicken files are often kept busy by a feature called "bill minder". These busy files (files ending in .QDF, .QTX, .QSD, .QPH, and .QEL) often cause the backup to error out.
Quicken and QuickBooks files should be backed up by using the backup option in the application rather than trying to backup the raw files.
See also Knowledge Base article 195551 - Maxtor Backup supported filetypes.