Why do you want to do it this way, rather than simply install the new HDD in the optical disk drive caddy and leave the SSD where it is?

@gronostaj because the optical disk slot is faster and I would like to put the SO in the faster drive. for this reason I just want to change the position of the existing SSD

Is it actually? Usually it's the other way around: main bay is SATA3 and ODD bay is SATA2 because optical disks are way, way slower. I doubt Apple would use an unnecessarily fast link for ODD and a slower one for main bay.

@gronostaj so why does everyone install the boot disk into the optical disk and say it's faster?

@gronostaj I have read something about it and I think I will follow your suggestion. I leave the crucial SSD in its slot and instead of the optical drive I insert a 1GB Segate SSHD (W7712E9X). I thought I would do either a FAT 32 or NTFS format for this SSHD to use as storage data

@gronostaj or install another crucial mx500?

Are you using macOS, Windows or Linux? What are you going to use it for (more specifically than "storage")?

@gronostaj I only use macOS and rarely a virtual machine. The new disk is for storage only

What kind of storage? Large files, like movies or ISOs? Many tiny files? VM disks? How are you going to use these files? Why do you want to use FAT32 or NTFS if you don't need portability?

@gronostaj the purpose of the second disk is that of data storage and I will also put virtual machines. maybe I would like to make two 500 partitions and one dedicated to backup the SO, even if I hate "Time Machine" because it doesn't allow a selective backup

@gronostaj I was planning to create a 500 APFS partition (backup) and an NTFS (data only) partition because the partition is more easily recoverable in case of failure

If you want to run VMs off that disk, I'd go for an SSD if your budget allows for it. It will make a difference.

I think your backup plan is not ideal. I wouldn't consider 2nd internal drive a proper backup. What if you lose your laptop or it's stolen? Both your data and the backup are gone. What if the power delivery section on the motherboard fails and fries both disks? Etc. etc.

Backup should be stored on an external medium. A USB HDD is a good starting point. If you don't want to connect some cables every now and then to update the backup, consider buying a NAS unit. It's basically a small, always-on storage server that you connect to over the local network.

Ideally, you should have a 2nd copy of all backups in a secondary location (friend's house etc.). That gives you resiliency against a house fire that would destroy both the laptop and the original backup drive. It's also a secondary copy in case of hardware/filesystem failure on the primary one. Decide yourself if your data is really so irreplaceable that you need a secondary. Cloud is also a good candidate for that one.

If you decide to go with a large internal backup disk (which I don't recommend) I'd use APFS for the entire drive. It's native for your platform and avoids the need to split the drive into fixed-size partitions. For an external drive I'd choose exFAT. It doesn't have the limitations of FAT32 and unnecessary overhead of NTFS.