Not sure quite which you mean; the BIOS item actually called "Legacy" is "Legacy USB storage detect" (later BIOS say "USB Storage Function") - either way, set to "Disabled" unless you, on the next boot, need to access a USB stick from the BIOS for flashing, storing <F11><F12> 'storage slot' parameter sets to USB, or booting from USB - it's prone to causing problems (especially 'reboot loops') if left enabled at other times...
The usual SATA question is re "Native Mode"; this actually simply enables or disables interrupt sharing for the SATA controllers - as the last version of Windoze that didn't support IRQ sharing was MilleniumEdition (which is likely a collector's item by now - I think they only ever sold eight or so of 'em!), for any current system, you always want this "Enabled"...