Moving from Windows 7 to Windows 10

If it's secondary drive without OS you don't have to format it or do anything to it. If you are installing W10 on primary HDD, disconnect secondary one so there's no confusion of where to install windows (or any other OS) to it It's safer and easier that way although not a must. After finishing W10 installation to primary disk you can just reconnect second disk and it will be recognized and usable in full although it may not have same letter but that could be changed later on. Only C: partition can't be changed.
Upgrade from W7 to 10 still possible but if you are doing clean install, you'll be presented with possibility to delete all the partitions on primary drive so windows can make new ones as needed. That part works same since XP.
If you want to attempt upgrade, do it from mounted ISO and choose Setup.exe. That should preserve all your setting and programs from W7.
If you have bought W10, you'd be better off doing clean installation.
For all of that you have great guides in Windows 10 Help Forums

Hi, I would be concerned that you don't have backups.. or so it seems.

Some people who upgraded to the 1809 build of Windows 10 found data lost- it's a big issue and MS has withdrawn that build now (Win 10 has a new build released every 6 months, with updates between - completely unlike Win 7).

Please make sure you have a complete disk image of your present PC before attempting any upgrade. I had one incompatible program installed that meant my PC was unbootable afterwards- I had to restore my disk image.

Strongly recommended here is the routine use of Macrium Reflect (free/commercial) + large enough external disk for disk image sets (typically base image + a set of differential images for each partition or partitions selected for imaging).

Disk images are normally compressed copies of the used parts of the selected disks or partitions.

A USB3 2TB disk might cost £55 or equivalent- 1Tb £40 or so. (I've not checked exact current prices).

You would expect so.

However some people have found data disks being seen as RAW - quite a few threads on that.

If you've clean installed '10 and intend to use that disk on a sytem previously with a '7 license, please see

Clean Install Windows 10 Directly without having to Upgrade First | Windows 10 Tutorials
for reference - license issues.

Perhaps you've already dealt with that though, and hopefully you installed Win 10 with AHCI protocol enabled (better SSD performance) if supported by your PC.

See what happens, then search the forum for such threads and solutions. (Or you could do the reverse of that for reference...)

Frano35 said:

Came to report that, when I turn off pc and plug in hdd with data, windows asks which os it wants to boot and only offers windows7, but windows 7 is installed on other ssd. So i switch ssd's and plug in hdd, pc works fine with windows7, then I copy some files on external HDD and when i switch back to windows 10, i don't see those files on external drive. Any thoughts of that?

How did you install W10 ?