Thanks for the effort, Mario.
I tried 2.55 and got the same behavior as I did with 2.53. Sorry!
On the other topic, as I said, my externally powered hard drive was formatted as FAT32. I have checked and my thumb drives always are FAT32. I do not know how to make them VFAT. If that experiment is still of value, I will try to figure out how to format the external drive as VFAT. If you have any hints on how to do it, they would be appreciated.