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I have a Sansa M250 with the latest firmware updates. I have it in AutoDetect mode and I copy all my files onto the M250 by draging and dropping in windows.

Untill recently I’ve had no problems with it, but now I sometimes get the message: 

"[file name] could not be placed on the device. You do not have permission to create this item"

I was able to delete everything off the the device and copy and paste about 1gig worth of files in folders onto it this morning. Now I try to add one tune to a folder and I get the message above.

I’ve found that I can copy the file to the main directory but, once on the device I still can’t copy the file into the folder I want it in.

Do I have to delete the folder from the M250 and the add the files I want to it on my pc and then copy the entire folder back onto the M250?

are you using MTP/Autodetect?  try transferring on MSC.