I heard/read that tixati handles long file names properly.

I am not sure what that means but I have an issue, on windows 7.

I have my main tixati seed folder on an internal HDD as F:\BT

I want to try and transfer my torrent file serve operation to an external HDD and make it independent from my computer, so it will run the portable version of tixati (and fopnu and darkmx) from any available computer.

To start, I used windows explorer to copy all files and folders in F:\BT to my external USB HDD at X:\BT, so same path length.

Windows balked with numerous files, saying the file name was too long for the destination drive.

Both drives are formatted with NTFS. So why the error?  If the file in on one drive apparently with a long file name, why is it not allowed to copy the same file, same length to the external drive?

You have heard this right, Tixati supports long file paths, that's why you have them there. Windows Explorer does not. Most other applications do not! Try TotalCommander to move these files, it has proper support.