>Yet as soon as that is done, the MacOS partition is trashed!!  I start 
>getting
>weird errors trying to launch things, and if I "Restart", I get a "?" floppy
>icon because the boot volume is damaged!   I then boot from a System 7.5.3
>CD-ROM and I get a popup saying "Could not open the disk/volume La Cie 1250-Q
>due to an error of Type -127" or somesuch.  Disk First Aid says "There are
>problems but I can't repair them".  Trashed!  I'm now up to my 4th reinstall
>of System 7.5.3 off of the CD-ROM!  Arrggh!  Why did the "mkfs" (apparently)
>go running roughshod over some vital MacOS blocks???

Mkfs has a problem addressing any disk block above a certain point.  It 
only uses a 6-byte CMD for talking to the SCSI Manager which limits it to 
21-bits for block address. When it exceeds this is happily truncates the 
numbers causing them to wrap down to lower numbered blocks. So if in you 
example you tried to run Mkfs on the 900Meg Usr Partition, it would have 
clobbered the MacOS Partition and maybe the Driver and Partition Map as 
well.  Since you got a "?" in the reboot it's safe to say you luckily 
didn't zap the Driver Partition. If you had you'd have gotten a Sad Mac 
and a very sad sound on boot. (Been there, done that.)

I have a "fixed" Mkfs that doesn't stray beyond 21-bit block addresses 
you can try if you'd like.  And if someone can describe the 10-byte or 
12-byte CMD buffer for addressing larger disks I'd be happy to make the 
change.  In the meantime you might try justing Mkfs'ing your Root 
partition and loading NetBSD there.  Then run Mkfs under NetBSD to get 
the big USR Partition.

-bob