I’ve find the solution for this problem.
We want to use MTP mode because this is the only mode that allows drag and drop to the external uSD card in the Sansa. In MSP mode the Sansa appears as mapped drive in Windows explorer, but the external uSD card does not show up.
I found the problem by looking at the folders that it had corrupted the folder name for. I noticed that it only seemed to be th elonger ones. After a few experiments I have found that the maximum folder name length is 28 characters. If it is any longer than this then the Sansa will prefix the name with an 8 digit hexadecimal string followed by a hyphen followed by the next few characters of the folder name.
The problem only happens when dragging folders onto the external uSD card, not for the internal FLASH of the Sansa.
I tried renaming the folders to shorter names on the Sansa drive in-situ (via Windows Explorer) but that didn’t work.
First you must delete all the folders that this has happened to on the Sansa. Next rename these folders on your Windows PC so they have a maximum of 28 characters. I chopped off the end and placed a ‘-’ at the end to indicate they had been shortened.
For example, a folder called “Jesus and Mary Chain - Psychocandy” was renamed to “Jesus & Mary Chain - Psycho-”
Next drag these shorter folders over to the “external uSD card” folder of the Sansa. Note that the “corruption” of the folder names doesn’t occur when the folders are dragged over, but when the Sansa does the “Refreshing your media” process when you disconnect it. Luckily the filenames within folders are not affected.
That should solve your problem!
I don’t know if this is a bug in the Sansa, or a consequence of MTP mode. I suspect the former since dragging to the internal memory works fine, and the corruption only occurs AFTER the dragging operation.