Use a backup program that transfers your files to a different computer using TCP/IP.

And make sure it is a backup program that handles multiple generations of files, so that a transfer of an infected file will still leave you with older versions of the file before the infection.

A good program would show you a list of modified files, quickly indicating if unexpected files gets backed up as modified.

But for a minimalistic approach, I use the script-based tool "backup2l" (backup-tool) to maintain a generation-based backup. The produced files are compressed and can restore (with my setup) any file on a daily basis for a 9-18 days. Then every 9th day for for 12-23 weeks. Then every 3 months for 2-4 years. And then every 2 years up to 6 years back. Only think I can't backup directly is the locked registry files. backup2l does it's job by having four levels of files. Full-backup files. Next level is a delta-backup every 3 months. Next level is a delta-backup every 9 days. And fourth and last level is a delta-backpu every day.

The reasons why people aren't using tape backups anymore isn't just cost. The problem is that the tape technology isn't progressing as fast as the disk technology. As a private person, you can't afford to buy a tape drive suitable for backing up a single 2TB disk. A company can buy such a drive with tape loader, but will not have any use of it since the company instead needs a mangitude more capacity.

But you shouldn't be too confident in the security of a tape drive - most drives have drivers making them accessible through standard OS API, making it easy for a virus to format a tape that is left in the drive. A dedicated backup software may have proprietary file formats when taping information, making for example the standard Windows backup application unable to restore, but you don't need to know how the files are stored to erase the tape.

A RAID is good for availability, in case of hardware failures. But it ins't even close to being a backup. A proper backup requires that the data is stored in multiple generations at (at least one) an off-site location. Backing to a dedicated backup server in a different building can allow the backup server to automatically write-protect and hide previous backup generations. With a backup client that tunnels the data, a virus-affected machine will not even know about the existence of the backup machine.