eSATA raid controller external drive problem error

I have a PCI eSATA raid controller in my classic Mac Pro '09 running El Capitan which I am using in JBOD mode with an external 4 drive enclosure and also for a OS 10.8.5 external boot drive for running some essential legacy software. In El Capitan I keep getting this error even though I have neither ejected nor turned off the Mt. Lion boot drive (in OWC single drive enclosure):

Disk Not Ejected Properly - Eject "Mountain Lion" before disconnecting or turning it off

The same drive, when...

eSATA raid controller external drive problem error