[@Alan] Your question was how much space is needed on your backup drive? I don’t know this particular software, and whether it compresses the data, and how well it compresses. It may also create some sort of index or datafile of checksums to help if there’s a problem on the backup drive. Also don’t know if it creates an exact image (so any unused space on your hard drive is also copied over), or how full your hard drive is. So, as a guess, I’d say you need anywhere from .5 to 1.5 TB for a 1 TB drive.
Also keep in mind, it’s very nice to be able to make more than one image. So, for example, week 1 you make image A, week 2 you make image B, week 3 you write over image A. Week 4 you write over image B. Keep alternating. Much safer. Best is if image A and B are on two separate drives in case one goes bad. They can be on the same drive, it all depends on how valuable your data is and how often it changes.
I use Zools to send recent changes to a cloud backup, very critical changes I copy right away to MS OneDrive, and I use Macrium Reflect (free version is great) to do a more complete backup. Macrium does not spend time copying unused space, and it can do full backups of drives or partitions, plus incremental backups where only things that are different from the last full backup are included in the backup. You can “mount” a backup disk and read or copy any file from it, or you can restore the whole thing. It’s got a steeper learning curve than the O&O program offered here, but I expect it will save a lot on space and time. I have a 6 TB backup drive, 3.5 TB of storage on my PC across two hard drives and an SSD. Backup drive has room for 2 full backups and 2 or 3 incremental backups. That’s just my experience, yours may be different!