Cant get space to load new contents in my device

I have problems with the capacity of my Sansa Sandisk m250 MP3 (2G capacity). I delete music files to get space to load other files, but nothing happens, there isn’t more space free in the device. I delete, appears in the computer information about space available, but when I try to transfer files from the computer to the MP3, it appears that there is no enough space, so I can’t load new contents in my MP3. Additionally, when I plug the device to my computer - a Mac Os- have been appeared files that not appeared before: audact.tcc, mpm_usb.ini, sys_conf. dnc What can I do??? I’m desperate with this situation!!!