I've used an external IDE enclosure from Kangaroo with an ATA disk of 128 GB with USB 2 and FW 400, formatted as HFS+ for years without issues.
I connected it via FW to an MDD with OS 9.2.2 and via USB to an XP SP3 laptop with MacDrive Pro 9.
I updated to Windows 10 Home. It asked to reformat the HD. I chose GPT because I was curious what the effect would be. Since then I can't use it with FW on the MDD.
I reformatted it several times to remove the GPT and I tried different formats, FAT32, HFS, HFS+. I can use it with the MDD via USB but not FW.
It doesn't show up in Apple System Profiler.
You can guru, you can! What am I to do?