memory recognition?

[Gloria Trias]

Why won't my Mac allow me to transfer files to my pen drive or my son's mp3 when they are both empty of all files? The message that comes up says
"xx file cannot be transferred because there is not sufficent space". This is particularly galling in the case of my son's MP3, a giga of memory, he got for Xmas. He downloaded some songs, max 100, wasn't happy and deleted them all. Tried to transfer some more, and got this message again.

Appreciate any pointers, thanks!

Gloria

[DeltaMac]

When you delete files on a USB flash drive, these will remain in the trash until you empty the trash. Because they are in the trash, they still continue to take up space.
Put the flash drive in the Mac, then empty the trash.

If this does not help, then open your Disk Utility (in your Applications/Utilities folder) Select the flash drive, and click the Erase tab, then click the Erase button you see on that screen.