usb memory stick and ext3

[Vu Pham]

I have a client who uses usb memory stick for backup of their database. 
The memory stick is formatted as ext3 and mounted then database is 
copied over at night. Everyday the same set of files ( same names  ) are 
copied over.

I notice that for an empty stick, cp copies files at a reasonable speed 
but for a stick that already has data, cp runs extremely slowly. After a 
reformat ( mkfs.ext3 ) the cp runs fast again.

Any reason for this ? Anyone uses memory stick for backup ? Any 
recommendation ?

Thanks,

Vu