Yes, it is possible. There are several tutorials on how to create a bootable USB. The directions can have some distribution specific items (e.g. Suse has some custom scripts that are used to create a USB boot from the live CD) in them so you will want to find (google) some for your install. You mentioned CentOS in your subject: here
is a link.
Once you are running your 'liveUSB' you will very likely have access to the hard drives automatically via the file browsers. If you do not, you can manually mount them and even add entries to your fstab to make the mounts permanent.