How to backup Quicken / QuickBooks files
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.
Open Quicken or QuickBooks.
Click File on the top menu.
Point the files to your external drive or somewhere other than the drive where the main files are stored.
See also Knowledge Base article 195551 - Maxtor Backup supported filetypes.