Please read this entire missive before beginning. Also, I must state that I assume no liability for anything bad that could (unlikely) happen.
I recommend beginning by booting from USB installation media. Once you get to the first screen, use shift-F10 to get to a command prompt and run diskpart.
diskpart> list disk
Disk ### Status Size Free Dyn Gpt
-------- ------------- ------- ------- --- ---
Disk 0 Online 1907 GB 0 B *
Disk 1 Online 1024 GB 0 B *
Pick the 990 by:
diskpart> select disk y
Disk y is now the selected disk ; where y is the # of the 990
diskpart> list partition
Partition ### Type Size Offset
------------- ---------------- ------- -------
Partition 1 Reserved 128 MB 17 KB
Partition 2 Primary 100 MB 129 MB
Partition 3 Primary 1906 GB 229 MB
Partition 4 System 100 MB 1906 GB
Partition 5 Recovery 832 MB 1906 GB
If you see a partition with the type SYSTEM, select it. If you don't see a SYSTEM partition, skip to the bottom of this tutorial for how to make one. Afterwards, resume from list partition above.
diskpart> select partition 4
Partition 4 is now the selected partition.
Now assign an unused drive letter. I'll use z: for this purpose:
diskpart> assign letter=z
DiskPart successfully assigned the drive letter or mount point.
diskpart> exit
Now back at a regular drive letter prompt, type the following:
bcdboot c:\windows /s z: /f UEFI
bcdboot should tell you that it transferred the files successfully. Afterwards, you can go back into diskpart, list volume, select the volume with the drive letter z: and type
diskpart> remove letter=z
DiskPart successfully removed the drive letter or mount point.
diskpart> exit
Afterwards, if you want to clear the 980, pick it:
diskpart> select disk x ; where x is the 980
diskpart> clean ; this command will remove everything on the SSD, so make sure you're doing the right one. You should get a message telling you diskpart was successful.
You may need to set the correct boot in the UEFI BIOS, especially if you didn't clean the 980.
***If you don't see a partition with the type SYSTEM, we'll need to allot the space for it. This example is using the partitions listed above:
diskpart> select partition 3
Partition 3 is now the selected partition.
diskpart> shrink desired=200
diskpart successfully shrunk the partition by: 200 MB
diskpart> create partition efi
diskpart succeeded in creating the specified partition
diskpart> format fs=fat32 quick
diskpart should tell you it formatted the partition successfully. Resume from above.