I understand everything, but only one answered my question. For a hard drive backup, which file system is "best" ? ext4 ntfs or ufs. The problem is that I don't know how to see UFS from windows
The reason I haven't answered your question is that I don't know a good answer.
What you're trying to do is to have a single file system (ext4, ntfs, ufs, ...) that is accessed from two different operating systems, one at a time. The problem with that is that file systems are deeply integrated into the OS they are developed for: Ext4 is native on Linux, NTFS on Windows, and UFS on the BSDs. For using it on a different OS, you'll have to use a "compatible" file system implementation, which is always a second class citizen, not written by the original developers of the file system, and in the case of NTFS written without full access to documentation and a sample implementation. Modern file systems are heinously complex, and have very interesting internal metadata structures, which are hard to keep correct. Or to put it bluntly: those alternate implementations are junk, and likely to cause corruption and data loss.
Here are two versions my advice: If you can, use that second disk and file system primarily from one OS, with the native file system implementation in read-write mode, and the alternate one in read-only mode. So for example, if you decide that you use Windows more often, make the backup disk NTFS, and from Linux and BSD only read it, and be carefully to check that what you read is correct (like manually verify sizes and checksums). Even better: GIve the second disk its own computer, which runs continuously. You could for example get a very small, inexpensive and power-efficient server (Raspberry Pi?), and attach the backup disk (formatted as Ext4 to match the Pi's Raspbian OS) to the network, and mount it via NFS or Samba. Or you could keep the disk in your main computer, but whatever primary OS you are running, also set up a VM with a small OS kernel that is used only as a disk server, and again exports the disk via a network protocol.
But in reality, we're discussing an XY problem here. Let's analyze your basic problem: You want reliable backup, and you want that backup to be accessible from multiple OSes. Why are you trying to solve that problem yourself? Many other people have long solved it, and they are certainly more of them than you alone. And without undue criticism: they are probably much more experienced in storage than you are. Simply move your backup to a cloud provider. There are many potential solutions, such as Amazon, Backblaze, Dropbox, Google, and smaller one. Any of those solutions is going to be easier and more reliable than something that you cook up yourself.