1. I suppose you have good & sufficient reasons for your proposed revised configuration. Frankly, the proposed revision seems rather odd to me. I would be interested in learning why you want to pursue this type of configuration if you care to share this information with us.
2. In any event, you've got to provide more detailed information re the drive's present configuration and the volume of data involved.
3. It would be best - virtually essential - that you post a current screenshot of Disk Management, ensuring that you include in the screeenshot the top portion of DM reflecting the detailed info of the data listed under the "Volume" to "% Free" headings.
4. Do you have another HDD or SSD available that you can connect to the system either internally or as a USB external drive?
5. As far as your IRST problem, check to determine whether a BIOS update or updated IRST driver has been issued for your motherboard or OEM PC.