Linux

If "D" is physically on the same hard drive, then you'll probably want to back it up before installing. Technically, you can manage to do it without screwing everything up, but I would not trust myself to. It's always a good idea to have backups anyway.

Also, user files typically reside on C by default and it takes some effort to put them on a different drive. Things like Downloads, Documents, Pictures, etc. so it's worth checking that before wiping as well.

Additionally, you'll probably want to format your "D" drive to a Linux native filesystem (eventually, after you back it up, because formatting results in data loss). While Linux does support NTFS quite well, it's not perfect, and your data would probably be safer on ext4 or f2fs (depending on if you have HDDs or SSDs) (or zfs or btrfs is you're into COW filesystems).

In Linux, you have all of your files mounted to a single "drive" called /. Everything is below /, which is called the "root" of your filesystem.

Typically, user data is stored in "/home" and this resides in the same directory structure as the rest of your OS, but on most systems it's on a different filesystem or even on a different drive entirely. This is because in Linux it is routine to put a "D" drive just in a folder. On my computer, I have several of these mount points defined, so the different types of data don't get mixed around, and I don't have to worry about downloading too much bullshit affecting my computer's updates.

Hope this helps.

@OmnipotentEntity makes a good point. Most (and definitely older) laptops have 1 drive. Which would mean your C and D 'drives' are actually 2 partitions on 1 physical hard drive. This is fine, but you need to be extra cautious when installing Linux. Many linux installers push you to the easiest choice and select 'wipe whole disk and install linux', which in your case would possibly lead to inadvertently wiping the D partition too.

You might want to pay extra attention to this during the installation, when selecting which disk to use for installation. Make sure you only let the installer delete the C partition (which will probably not be named as such, so be ready to find another way to identify the correct partition (maybe by its size?)), and let the installer use the free space that gives to create linux partitions it needs there (next to the D partition).

NB: Still in Windows, you may also want to check whether or not your D drive is encrypted with Bitlocker, as that is a Windows-type encryption and cant be unlocked without a recovery key (aside from it not being practical to use Bitlocker encryption in combination with linux (or NTFS for that sake, as OmnipotentEntity also already mentioned)). If so, you might want to decrypt the D partition so you can still access it from Linux (while it is of course better to have encryption enabled, it may be a temporary convenience).

Edit: Solid choice of Linux Mint btw. It's been a while since I've used it, but in my memory (also as a starting Linux user) it made the right things easy. If your laptop is quite old, and Cinnamon (also solid) doesn't feel quite snappy enough, you could give XFCE a try. It's less polished (some say ugly 😆) out-of-the-box, but also less resource hungry (Cinnamon and XFCE are both Desktop Environments (DE's). On Linux you can have multiple DE's installed side-by-side; and then make a choice which DE you want to use when you login)

That's a good MO to have. I was talking about the partitions that Windows does on its own, depending on the version, of course, and the ones that the manufacturer does. For example, Lenovo has a recovery partition with a (supposedly) untouched installer for the Lenovo bloatwared OOTB Windows installation. Since you never plan on using that laptop for that version of windows again, you can just merge all partitions on that drive, and let the Mint installer use the whole drive for its purpose.

Thank you for your extensive answer, I feel confident now to do the "leap of faith"