My Book Live very slow

I have MyBookLive WDC WD30EFRX-68AX9N0. When I used 2.9 TB of the available 3 TB, I got an alert that disk is almost full and that I’m going to experience slow performance until I free up some space. I did notice that performance significantly deteriorated, particularly when copying a lot of small files to the drive. I then cleared some data and the used space is now 2.6 TB, but performance did not improve. Any help will be appreciated.

Check HDD condition,

on my book live WDBVHT0060JCH one of the hard drive died, how can i setup RAID using big sur

How?