It seems your problem is not with MS or BC or USB drivers, it's with the device itself or how Windows is treating it. StreetPilot 2610 does not connect in Mass Storage Mode in Windows, so i don't know why your BMW unit is being recognised as such and given a drive letter [E:] by Windows. It should not be visible as a removable device at all. It would normally be seen by MS & BC directly without it appearing in Windows Explorer/Computer or Device Manager/Disk Management.
I suppose you have already tried cold rebooting your PC, then reconnecting the GPS after it's powered up, and finally opening MS or BC.
This is how SP 2610 should appear, only shown in Devices & Printers without a letter:
GARMIN (H:) showing in Disk Management above is not the SP, it's a 4GB logical drive on my main HDD.
I have had no direct dealings with the BMW II navigators, but i believe that you are correct when you say the hardware is the same as SP 2610. There are slight differences in the firmware only afaik.