If you have a SATA hardrive then your BIOS may (by default) see this as part of a RAID array.
RAID arrays allow the storage of all your data across several hardrives for data security (raid 1 where data is duplicated) or access speed (raid 0 where data is spread alternately across 2 or more hardrives).
If you only have one hardrive and everything working ok then leave it alone.
There are other versions of RAID (2 and above) but these are for server systems and impractical for home use.
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