compare Windows folder with smartphone folder

I know this post is two years old but I ran into this issue recently and I think it could be useful.

I found the FreeFileSync software from freefilesync.org that supports the MTP protocol natively.

So if the folder is on my SD card, it would be: mtp:\Xperia Z5 Compact\SD Card\Folder to sync

It appears this is an issue with lots of comparison tools and smartphones, and has been for a while. As you guessed, the main issue is that phones attached as storage are not assigned drive letters when they are connected (See here for a thread dealing with the same issue from nearly two years ago: http://www.scootersoftware.com/vbulletin/showthread.php?9696-How-to-open-a-mobile-device-%28smartphone%29-for-compare-with-no-drive-letter-assigned)

One workaround is to use a comparison tool that supports FTP connections, then set up an FTP server on your Android phone and run the comparison that way. It will probably be much slower than a USB connection, but still better than nothing.

In order to actually get a drive letter assigned to your Android phone, you'll need to not use the MTP connection method, which means rooting your device and using one of the various methods available to mount your phone as mass storage (https://android.stackexchange.com/questions/22979/drive-letter-for-mtp-connection-under-windows).

A long term solution is to use a different tool for synchronizing files between your phone and your computer.

Personally, I use Bittorrent Sync to move photos from my phone to my computer. When I remove files from the synced folder on the computer, they are moved to a hidden folder on my phone as well, and then I just delete them from there knowing they've already been dealt with on the computer.