Disk ][ vs Unidisk vs Duodisk vs //c external disk

[shoppa@alph02.triumf.ca (Tim Shoppa)]

Starting 1979 or so the only Apple II disk drives I had ever used
were Disk ]['s on ]['s, ][+'s, and //e's.  I've recently begun
running across quite a few drives with 19-pin D connectors:
Unidisks, Duodisks, and //c external disks.  I've also begun
entertaining the thought of buying a IIgs.  A few questions:

1.  Am I correct in assuming that I can just daisy-chain these
drives onto each other?

2.  I understand a IIgs can control up to four drives in a
daisy-chain.  Is this also true for the "Duodisk" controllers
I've been seeing in //e's?  Or are these limited to a total
of two drives (a single Duodisk?)

3.  What is the wiring of the 19-pin connector?  How is
drive select handled?  I understand the 20-pin Disk ][ cabling
quite well, but don't see how it is easily extended to include
4 drive selects.

Thanks!
Tim. (shoppa@triumf.ca)