If your Tandy 20 meg hardcard has a Miniscribe model 8438
drive on it, then you have one of the group (like mine) where
Tandy mated a 30 meg drive with a 20 meg controller.  MFM and
RLL are anacronymns for two methods of encoding data (I don't
remember what they stand for).  20 megs will be MFM, and 30
megs will be RLL for drives this size.

Swapping the Western Digital controller for an OMTI 5527-10
controller will net you ten more megs, plus about a 30%
reduction in disk access time.

Keep in mind that Tandy doesn't make either the drive or the
controller, and might not even make the frame on their
hardcards, so you are under no penalty if you buy it
somewhere else.

Anyway, the place to buy the OMTI controllers is Sunnyvale
Memories.  They are in California, and their sales number is
(800) 262-3475.  Now if your computer is a 1000A, SX, or TX
without the video memory expansion, then you need the special
Tandy BIOS they offer.  It seems that this controller sets up
a 1K memory cache, and it normally puts it in the top of
memory, which is where the 1000's put their video.  The
special BIOS puts this cache in a different memory location.
This model is the 5527-10T.

If you are not using the on-board video in an SX, or you have
a TX with the expansion or another video card, then get the
standard version, model 5527-10.  The only difference is the
BIOS chip, and the bracket.

The standard version is $85.00, and with the special BIOS is
$125.00.   If you get the special Tandy version, ask them to
toss in the standard BIOS, in case you ever upgrade.  They
did this for me, and it turned out to be only 3 months before
I switched machines!

By the way, the SmartWatch works fine mounted under the BIOS
on the OMTI controller, and doesn't interfere with the cables
like it did on the Western Digital Controller.  I have the
interleave set to 2:1, but the cache seems to distort this
since I get the same access times at 1:1, 2:1, 3:1, or 4:1.
You will have to reformat the drive using debug, but the
instructions are pretty good.

Rich