Strange, very strange!
I have been using SyncToy 2.1 for years to backup 4 folders to an exterior USB docked HDD. As those of you who have used Synctoy know it pairs an original folder with a folder in a backup location and sync the backup folder with the original. (there are 3 schemes one may chose form I only use ‘echo’).
This has worked just fine for years, I have two drives I use always setting them to drive M in disk management if need be as windows tends to assign D to one or the other of them. I have two sets of 4 folders pairs, one set for each drive.
Yesterday I went to do my usual sync and got an error message from Synctoy that said it could not find the target folder. (The jpg shows the wording of the message as an example). I checked and the M drive was present as M, I checked to see if I had used the correct folder pair and noticed some folder pairs were listed with the target as the same drive letter as my source drive (H) but pointing to a correct folder structure for the M drive but pointing to a nonexistent location on my H drive. I reloaded Synctoy and now all targets were point to the H drive.
I tried a repair install of Synctoy and a reboot to no avail. Checking further I found config files in C:\Users\xxxx\AppData\Local\Microsoft\SyncToy\2.0. (%localappdata%\microsoft\synctoy\2.0). One called ‘SyncToyDirPairs.bin’ seemed a likely place to investigate. Indeed looking in text mode there were ‘H’s where there should have been “M”s. I edited the file but it did not take. There is no option within the program to change drive letters. I am resolved to recreate my backup scheme but am still curious how the seemingly impossible happened.
The MS d/l site lists improvements, one of which seems a possible culprit: Dynamic Drive Letter Assignment: Drive letter reassignment will now be detected and updated in the folder pair definition. ”
I made my way to the help file ( yes only 11 years ago there were still helpful help files !!). Dynamic Drive Letter Assignment is supposed to accommodate
changing drive letters but in this case seems to have stuck on the wrong one. I imagine if I had the same backup directory on my H drive it would have happily backed up to the wrong location.
This seems to have been a known problem.
https://social.microsoft.com/Forums/en-US/b5846da8-220d-4d3d-a3c3-cc2c0ed6fb36/lost-all-folder-pairs?forum=synctoy
Dynamic Drive Letter Assignment had never worked correctly as far as I know. I would like to turn it off or run the commandline version of synctoy.
Regrettably only one folder pair can be specified with the command line synctoy. It seems to be possible to use the GUI version with multiple folder pairs. This might actually save some time.
I know there will not be an MS solution I just want to let folk know about this problem. If I had lost my H drive and it had the directory structure Synctoy was looking for I would never have known I was backing up in the wrong location until that location (H drive) was lost to me.
BTW
I am considering the much mentioned ‘Karen’s Replicator’. 
