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RAID is when you have multiple disk drives, and some BIOS lets you configure them to act as one with improved performance or to add redundancy. It sounds like it prob isn’t an issue with a RAID controller. You could try another distro or loading GParted to see if it shows up there. I don’t have personal experience with LMDE specifically to know if there is something specific there, but for it to not show up at all is surprising.
Thanks! I solved one problem (SATA mode changed to AHCI made the disk appear) and quickly found another, so I may be investigating RAID solutions next. Appreciate the succinct explanation!