As with any hardware addition, first try the obvious (boot -r
after installing and power-cycling everything).
The adaptec is no longer supported; man -s7 sd no longer even
lists it! So I guess they go over the cliff. Either that, or
take the drives out and put them on a PC, where ST506 MFM
drives are still supported.
The MD21 should work, though some people report that SCSI
doesn't work in 4/260 boxes (bug-id #1118752), but that's
fixed in 2.3 and later.