Format option in context menu in Disk Management disabled

I want to completely delete all partition and format one of my drives. When I go to Disk Management and right-click the drive, I can't select "Format", nor "Delete Volume".


Then, when I try to use DISKPART from the command line, I get this message:

Code:

DISKPART> clean

Virtual Disk Service error:
Clean is not allowed on the disk containing the current boot,
system, pagefile, crashdump or hibernation volume.

Even though is said the drive contains the current boot, it does not. I have a separate SSD which have Windows installed, and which is my main OS drive.

I even tried to format the drive from Explorer, but got the error message: "The drive is in use by another program. Do you want to continue?" (something like that.)

I believe the drive I want to clean is somehow used to boot Windows or used by a program. However, trying to think of currently open programs installed on the drive, I come up short. I tried closing all open programs to no avail. Furthermore, I shut down my computer, disconnected the SATA cable to the drive and booted my machine up again. Windows booted up with no troubles.

The EFI partition could be from when I had Windows installed on the drive, but that's a long time ago now. The only solution I have at the moment is to boot up Linux from a thumb-drive and format the disk from there. However, I would prefer to do it in Windows just to be safe.

If anyone know what could be the problem, or could point me in the right direction, please do let me know! Thanks!

Edit: I posted over at Tom's Hardware as well. No solution is found yet, but there is a discussion going on. Format option in context menu in Disk Management disabled - Hard Drives - Storage