Hi there, Paul. I'm here to ensure you'll able to create a backup copy of your company file to an external drive in QuickBooks Desktop (QBDT).

To start, may I know if you receive an error when creating a backup copy of your company file? If none, let's first verify that you can see and locate the external drive on your computer. 

If confirmed, you can also create a backup copy of the company file and saved it to a local folder in your computer, and copy it to the external drive. Here's how:

1. In your QBDT company, go to the File menu and select Switch to Single-user Mode.
2. After that, go back to the File menu again and hover over Back up Company. Then, select Create Local Backup.
3. In the window, click Local Backup and then Next.
4. From there, select Browse and select a local folder you want to save your backup company file.
5. This runs a test to make sure your backup file is in good shape before you save.
6. Once done, select OK.
7. Click Save it now and Next. You'll get a confirmation message once done.
8. After that, move or copy the backup data to your external drive.

On the other hand, you can also use a different external drive when creating a backup. Might be that the current one you're using caused the issue. 

Additionally, for a company backup that is located on an external device, you have to move it first to your local hard drive before restoring it.

I also added this helpful resource that you can use as a guide in case you need your accountant to review your books while working: Use the Accountant's Copy. 

I'm always here ready to lend a hand if you have other concerns about creating backups of your company file. Have a great day ahead, and stay safe.