Number one:
When Garmin Express updates the SmartDrive to the newest map it deletes the old one.
You are saying after 2 hours of updates it replaces the old map back.
Sorry but that can not be.
If the SmartDrive 50 has WIFI.
The only thing you can do to try is forget your PC and just turn on the WIFI on the SmartDrive 50 and do the update that way.
You will need power to your unit so take a phone changer and the Garmin cable connect them and attach it to your SmartDrive 50
Go to Settings lower right hand.
Pick Wireless Network
Turn on WiFI
If it does not find your WIFI you can type in the name of your WIFI and Password manually.
Once you have done then come out of that window and then tap on Updates.
It will list the updates that are needed.
Save the map update to last.
This way there is no way it can restore you old map back on your SmartDrive 50 as you say that is what is happening.
If you still have problems then contact Garmin.