When you stopped using it for an OS drive, did you delete ALL of the existing partitions on the drive, or did you ONLY format the C: partition on that drive thinking that was all you needed to do?
It would be highly recommended that you back up anything important on that drive to someplace else and in either drive management or a third party partioning management program delete every partition on that drive including the hidden partitions from when it was a Windows drive, and then create a new GPT partition using all of the available space.
If you DID delete those partitions to start with, and created one single partition covering the entire drive, then I'd try doing a hard reset of the BIOS to reset the hardware tables. Maybe there's still a boot manager partition on that drive that needs wiped out or the record of it being on that drive in the UEFI configuration.
BIOS Hard Reset procedure
Power off the unit, switch the PSU off and unplug the PSU cord from either the wall or the power supply.
Remove the motherboard CMOS battery for five minutes. In some cases it may be necessary to remove the graphics card to access the CMOS battery.
During that five minutes, press the power button on the case for 30 seconds. After the five minutes is up, reinstall the CMOS battery making sure to insert it with the correct side up just as it came out.
If you had to remove the graphics card you can now reinstall it, but remember to reconnect your power cables if there were any attached to it as well as your display cable.
Now, plug the power supply cable back in, switch the PSU back on and power up the system. It should display the POST screen and the options to enter CMOS/BIOS setup. Enter the bios setup program and reconfigure the boot settings for either the Windows boot manager or for legacy systems, the drive your OS is installed on if necessary.
Save settings and exit. If the system will POST and boot then you can move forward from there including going back into the bios and configuring any other custom settings you may need to configure such as Memory XMP profile settings, custom fan profile settings or other specific settings you may have previously had configured that were wiped out by resetting the CMOS.
In some cases it may be necessary when you go into the BIOS after a reset, to load the Optimal default or Default values and then save settings, to actually get the hardware tables to reset in the boot manager.