QB Issue Resolution:

Double click the .dmg file to mount (open) drag the company file from the open window to documents>quickbooks, fro QB use file>open company and navigate to the file.

Resolution for Issue 'How do I make a back up of my Quickbooks file to a flash drive to take to another computer' available: Yes (Solved).
Source: Intuit Community forum.

How do I make a back up of my Quickbooks file to a flash drive to take to another computer: 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.

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