By this point, all things being equal, you should have either
- cleaned up an existing partition on one of those 2 TB drives (not /dev/sda) and formatted to EXT4 a destination partition ready to house your backup of Home folder or partition (you have yet to tell us which it is) or other Data-housing partition OR
- created in appropriately sized unallocated space a partition to either of which you have then
- backed up the data you need to save
If, and only if, that has been executed, then you are ready to
SUGGESTIONS (Cont.)
2. Write a new installation of Ubuntu onto /dev/sda, which process will delete all the RAID structure currently in place, and which is clearly proving to be more problematic, than problem-solving.
At this point, you should decide what version of Ubuntu to put on the computer.
- If you wish to put 21.04 'Hirsute Hippo" GNOME again, be aware that it will reach end of life around 20 January 2022.
- I would not consider 21.10 'Impish Idri' due for release around October 20 next, because it too has a nine-month shelf life.
- Waiting for the next LTS (Long Term Support) release to be available (known currently only by "JJ") will see you waitn until around 20 April 2022.
I would advocate instead putting on 20.04 'Focal Fossa', issued April 2020, but upgraded twice since. It is currently on 20.04.2 released 4 February last, so if you installed it, you would only be faced with 6 months of updates to follow up with, not 16 months.
Further, you have have 5 years support.
Let us know how you are situated currently, and what course of action you would like to follow.
Cheers
Wizard