My bad, I was miss-remembering what I read on it. It was a DIY kit to build a controller, and not a MOD.
So, it would take a totally new HW design to get HD on the TI-99/4A. From what I've been reading, there's nothing stopping the TI from handling it if the HW & DSR are set up right. I'm mainly just curious at this point. I want to replace the SSSD drive in my PEB with a standard 3.5" & 5.25" drive, and don't really want to worry about only using DD floppies (so the drive won't choke,) or modding the drive not to detect HD disks. That's the issue I'm facing with my Amiga. Get an adapter to use a PC floppy, and you have to either MOD the drive, Use only DD disks, or use tape to block the HD hole to prevent the drive from trying to go into HD mode (which the Amiga can't handle.) So, I have several options for installing a 5.25" & 3.5" drive, but they are all DD and not HD. I can live with that. Don't really want to MOD a 3.5" floppy (and finding a DD drive is problematic.) Now, I'm just faced with the fact, that none of those options are easy to find for sale (did a quick search.)
On the HDFC card, is it a HW issue or SW issue that stops the TI-99/4A from using HD drives? Is MyArc still around, or would it be safe(ish) to re-write the DSR & put them on EPROM(s)?