My suggestion: use two "external", USB-connected hard-drives. Then do a complete back up of the PC's drive to one of these external hard-drives. Next week do a complete back up to the second hard-drive.
Next week do a "sequential" back up to the first drive. This will back up ONLY files that are new or have changed since the last (in this case the original) back up.
The following week do the same with the second drive --- which will again ONLY back up changed or new files.
Many of the external USB-drives come with back up software. If both drives are identical only one of these application-software packages need to be installed on the PC.
With this scheme the last week will only be backed up on ONE back up hard-drive --- but all the other items are recorded on BOTH drives. If this is too risky, the sequence can be changed to shorter time periods, like back up to one drive on Wednesdays, to the other one on Friday / Saturday --- or any such combination.
This is a typical "industrial back up scheme" with alternate drives. For additional security the drives are then taken "off site" to a safe location, only to be returned for the next back up.
With the "right" software the back ups can be done without any further personal attention after the process has been started, thus can be started before we go to bed. After the back up has been completed (in the morning) the hard-drive can be shut off, reducing the operating hours / extending the life-time of the drives.