copy files

hi !
would like to copy all files in a usb key with 32 Go
but the system would not copy mail_data.dat with 6,9 Go because the system say it is too big

i do not understand why …
if somebody could give me an advice
thank you

Sounds like your 32 Gb USB has some hidden file that has reduced the storage available. Try re-formatting the USB device (use NTFS) to make sure it is all clean and empty. Then retry the copying of the mail folders.