It was initially set to use the letter E, which isn't used by any other drives.

Anyhow, I tried disabling hiding empty drives, and now I can't access the disk at all. Might be because chkdsk is...

I'm running chkdsk now.

CountMike, what does that have to do with the drive not automatically being assigned a drive letter by windows?

Aren't there any other options? The only spare drive space I've got, is on a very old and slow HDD, moving all my things there would take centuries

I don't know if you misread something in my post, but I can assign a drive letter through a third party software, and when I do that it works fine till I reboot.

The drive is internal, and the only thing I can do with disk management, is delete the partitions on it.

After formatting my extra SSD, windows won't assign it a drive letter when the computer starts. I have to use a third-party software to assign the letter, and I have to do it everytime it boots, and...