OK, so my understanding is this:

(i) You're sure you've loaded the cure fw correctly.
(ii) You have the latest Garmin USB Drivers loaded on your PC.
(iii)You're positive that Windows refuses to see your 1490 in Windows Explorer.
(vi)It's not displayed [even without a drive letter] in Disk Management [if you didn't check successfully before in Disk Management please do it again, not holding the screen, just plugging via a rear port as below].

Please now answer these questions as to your findings when the nuvi is connected to PC normally, [not holding the screen to access preboot] just plugging it to the PC via usb to access Mass Storage Mode. Ensure the connection is not via a front PC port, only to a rear port directly [not thru a hub] with the nuvi off:

(i) Does the PC give the connection tone shortly after you plug it in?
(ii) Does the nuvi display the Garmin logo with the PC tower image below it after connection?
(iii)Is any other info displayed below the logo, e.g. 'Loading Maps' even briefly?
(vi)How long before the nuvi self-disconnects and tries to reboot while still connected to PC?
(vii)If you insert a microSD into the nuvi, it that seen in Windows Explorer or Disk Management?