QB Issue Resolution:
You would have to run a transaction detail report, then export the report to excel, exclude transactions already in QBDT, reformat the excel sheet to iif format (this could be done with a third party transaction importer you purchase), then import to QBDT.
Typically the way it is designed is that you make a choice, use QBO only or desktop only.
Resolution for Issue 'Is there a way to backup my bank account activity?' available: Yes (Solved).
Source: Intuit Community forum.
Is there a way to backup my bank account activity?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.Support for Quickbooks
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