Hi,
First to change the H/D you wish to boot , not knowing what Motherboard you have nor the type of bios, many systems use the delete key to access the system bios. Start up your computer and while the system is still running it's system tests start tapping on the Delete key this should give you access to the system bios. It is in the resulting screens that you can change the order of devices your system will boot from. DON'T CHANGE ANY OTHER SETTINGS JUST THE BOOT DEVICE/ORDER!!! Read the screens carefully to understand how to change and save your new bios set-up. F1 Key usually used as Help This key is our friend and we should not be afraid of using it!! (Usually the F10 key to save your settings). Then the system will reboot using the order of devices you have input. Remember that you will have to either have an O/S already on the other H/D or must be ready to install a new O/S on it. Otherwise nothing will load and you will just get a warning that O/S is missing or the bios will just skip that H/D and go on to the first H/D it finds an O/S on.
Second are you wanting to Hide personnal folders/files or the actual System files?
The System files are usually already hidden and you have to have administrator priviledges to access them. Other wise if it's personnal files you don't want others to see or have access to just set up a personnal user account and use a private password to keep others out of your things.
Hope this helps,
Boozo