I have an usb 3.0 hard disk and anotherhard disk in the cd room bay. My computer recognises both normally, however, when I try to change the default save location in Tixati, it just doesn't let me go to my other two HDDs. I'm using the 64 bits version

If it detects in Windows, Tixati should be fine. You're going to which Location -> Move?
If on the other hand it was NOT detecting in Windows it could be a driver problem.
Start-Run -> "compmgmt.msc" go to Storage -> Drives .. if the drives show up and have a drive letter (C:) you may want to verify drivers :/

EDIT: AHH! On your Settings -> Transfers -> Locations ... check if you see all your C: D: E: etc if you double click on "This PC" .. The MOC settings may interfere, not sure.