Will a USB 1.1 cable transfer data (safely) at 2.0 speed?

If you are concerned about the integrity of the data, then why not sha1sum or md5sum and check them? The cable for 1.1 and 2.0 as far as I know are wired precisely the same way, differences probably being shielding and guage of wire. Frankly, I do not believe there will be any issues with your data, given the general length of most USB cables.