I'm curious: Why not use an IDE to SATA adapter for a build like this? When I read this I actually went searching to see if that exists and then I found a site that sells a modding service to add a SATA port to the side, but then they also sell a Dreamcast with this mod and the SD card mod so I'm thinking that it's probably not connected to the GD-ROM like you have.
Still, it should be possible to run an IDE mod to the GD-ROM and then add an IDE to SATA adapter to it. From there you could either use a 2.5" SATA drive (solid state even!) for an internal, or run an adapter to the outside of the case like they did.
It's a shame that a SATA to female USB doesn't exist though, because if it did you could take it a step further: Go from IDE to SATA to USB, allowing a flash drive to be interpreted as an actual IDE hard drive meaning you could actually add a USB port without having to destroy the GD-ROM like you'd have to do with the USB-GDROM mod. I wonder if it would be possible to actually build such an adapter though.
In my case, I ended up buying a modded Dreamcast from Dream Mods with a working GD-ROM, a VGA port, dual BIOS (one to load discs, the other to load Dreamshell from an SD card plugged into the serial port), an overclock switch, two Dreamconn+ controllers (which sadly won't work while overclocking) and two backlit Virtual memory cards that charge the battery from the Wireless controllers. I probably won't get the SD card reader for another month, so I hope the compatibility for my games is high enough. If not I may end up buying a second Dreamcast for the hard drive so not to have to ruin the GD-ROM. It looks like there's no room to add a mod like this though, which is a shame because a real Dream would have all of the above and a hard drive.