Best way to secure digital information?

If you are running a recent version of Windows (10 or 8.1) then BitLocker can be a good solution. BitLocker can be set up to use a key stored in the TPM chip on the motherboard. This woks fine for protecting disks when they are removed from the computer, but the disk is unlocked automatically when the computer boots. The solution is to use a password when you want to use access a disk. This can be used for the boot partition C: or any other partition, and you can use a different password for different partitions.

You do not need a PC that normally offers BitLocker. You can temporarily attach the drive as an extra drive to a BitLocker capable PC such as Windows 8.1 Pro or Windows 10 Pro, then BitLocker encrypt the partitions even giving a different password to each partition if required. If you encrypt the C: partition, when the PC boots from that drive it will ask for the password.

Put your personal data in a separate partition D:. If this has a different password from the C: (or C: is not protected) then you can let a repairer use C: but not give them access to D: and your personal data.

Another thing you might check out is using Virtual Hard Disks (VHD). These work in a similar manner to normal hard disks, but are contained in a normal data file. This makes it easy to backup a VHD or transfer them to and from a Windows 8.1/10 Pro system for managing BitLocker settings. This is the solution I use for securing my personal data.

I hope this helps.

Setup a server that does not connect to the internet and keep it in a secured room with limited access.