[Mommistake]

Insert your flash drive and let Windows mount it.
Open Control Panel
Select Administrative Tools
Select Computer Management
Under Storage, select Disk Management
Your flash drive will show up there, with the drive letter Windows assigned it.
Right-click it. Select Change Drive Letter and Paths...
In the dialog that pops up, select Change...
From the drop-down list that appears in the subsequent pop-up, pick the drive letter you want.

Note that if you already have A: assigned to a drive, you'll need to change its default drive letter first.

Once you have completed this operation, The local installation of Windows will always assign that letter to the drive you've assigned it to, any time you insert it. However, the assignment is only good for one computer. To use the same flash device as drive A: on another computer, you'll need to repeat the procedure.

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