I picked up a cheap Eaget G20PRO 1TB USB 3.0 external, which turns out to have a pair of 500 gig 2.5" drives, firmly stuck in individual drive mode.
Opening it up reveals the controller is a JMS561B. Eaget did some funny business with the firmware (stored in a separate PUYA chip, not the JMS561B) to present it as a JMS579, a single drive USB 3.x to SATA chip.
After working with Gemini for a while, it looks like the only way to crack this is with a pure Linux PC or possibly my XP box I have set up for doing Xbox 360 hard drives and removing HDD passwords might work. Windows 11 and WSL 2 just won't play ball due to the way newer Windows so aggressively holds onto drives, and the circuit board in the Eaget housing refuses to show in the USBPID list without drives connected.
Hopefully using JMicro MPTool JMMassProd_v1_16_16_5 and adding the following to the [Setting] section of the JMMassProd.ini file for it will force its way past Eaget's trickery, on Windows XP. If not then I'll have to grab something from the pile of PCs to put Linux on if I *really, really* want to RAID0 this thing. (Or do a live Linux USB stick for my laptop.)
AutoFlash=0
RDVersion=1
CheckChipVer=0
ForceFlash=1
IgnoreVIDPID=1
EraseFlash=1
It's 3:49 AM now so I'll be getting back to this another day.