SATA Boot Order

Can someone advice the boot order for connected SATA Drives ? Is there a
boot order ? Can it be set?

I ask because I have (or should say had) a XP SATA drive and Windows 7 SATA
drive both connected to SATA 1 and SATA 3 ports on motherboard - test bed
system.

System wouldn't start up, rebooted then options to recover or start normally.

disconnectng XP drive, Windows 7 would not boot (drive letter susepected
changed) and with only XP disk connected, it still wanted to boot windows 7
(I'd like an explanation of this !!!).

Result. Can't boot from either; looks like mbr corrupt on both. Tried to
fix XP mbr/boot sector but never got option to run recovery console. As far
as it was concerned there was no windows installation on disk???