Code:1001 volume usage is above 95% warning incorrect

Hoping someone can hlep.

I have a solid red power light and the above warning. BUT, this warning is for volume 1 and it is a 2tb drive and I can only locate 400 odd Gb on it by checking shares folders.

The dashboard has never shown the a correct number for the memory available even after starting OS5, it seems to only register one of the drives (likely volume 2). I have empties the recycle folder too.

Any ideas on ways to check volume usage and fix this error?

Thanks