The CUST DIAG tape's formatter is pretty much the same as the one the FE's
use other than that the FE version is prompt based and the customer one is
screen based.  The FE version costs hundreds of dollars but can be ordered
from DEC for those interested in a real "diagnostic monitor" like the bigger
machines have.  It's called MDM - the Microvax Diagnostic Monitor - and it
boots from TK50 in the incarnations I've had my hands on.

The reason your XT2190 isn't formatting is probably due to the age or jumper
configuration of your RQDX3.  Older ones have cylinder limits that stop
right after the expected end of an RD53, which was silly and later removed.
There are also some write-precomp things that were nec'y for the 10MB and
20MB drives, special-case-disabled for the RD53, but not for the RD54.  All
of this in hardware, as was DEC's forte.  Ick.  Anyway, a newer RQDX3 (M7555)
or a middle-aged one with the right jumpers will clean up your XT2190 woes.

I hate to think of how many brain cells I've got wired down with this data,
and the many better uses they could have been put to.