How to re-enable Windows user options pop-up for newly mounted USB Flash drives?

Go to: Control Panel > Hardware and Sounds > AutoPlay

Or more simply, click on Start > Type autoplay and press Enter

Put a check on Use AutoPlay for all media and devices to make that AutoPlay popup-dialog appears for you again when you insert a external media device.

You might need to reboot your computer in order for this to work.

When Autoplay system settings changes still do not work try the following to fix a corrupt registry and reset functionality:

Applicable for Windows 7-10 - copy the following into an editor:

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer]
"NoDriveTypeAutoRun"=dword:00000091

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Explorer]
"NoDriveTypeAutoRun"=-

and save it with the extension ".reg" (f.e. "autorun_restore_fix.reg") and run it - answer with yes. (Or run "Regedit" and edit the values in their proper keys as shown above). No need to reboot: changes to HKCU are immediate.

Hint: XP also can use the .reg file, just replace Registry Editor Version 5.00 with 4.00.

Alternatively, you can completely remove the following Explorer key from registry:

[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer]

...if there are no other values in it and you don't need specific users policies. Don't worry: there is no "Explorer" sub-key at default Windows installation, so it's safe deleting it. This will make the USB autoplay action requester pop-up again as well, resetting it to defaults.