UsbHubDriver: Warning: Connected high speed capable device to low-/full-speed controller.
UsbHubDriver: Root Hub USBHC0 (UHCI USB Host Controller): GetStatus request failed.
Is mass storage via a low-/full-speed controller excluded?
No. The warning just indicates that you have attached a USB highspeed device to a controller that only supports low-/fullspeed. USB highspeed devices are required to also support fullspeed operation - it is just slower.
In the screen shot, there is a message ".... is now accessible.". This means, that the USB mass storage device driver has been successfully bound to the device an that you can "access" the device using its name (in your example, USB0). It is not a guarantee that the device works without problems, however.
These lines appeared and of course the external drive was not mounted.
Have you tried to mount it? (accessible does not mean mounted)
Two machines yield the black screen and "Invalid Init" when config string editing is invoked.
I will ask Andre what the exact meaning of "invalid init" is. Most likely, the corresponding mode is simply not supported by your graphics hardware. To find out which modes are supported, you can use Andre`s Boot Manager.
Putatively, I can take the hdd from any troublesome
target machine, connect it to the Toshiba via an ATA-USB
adapter and install A2 that way. Will the target machine
boot?
There is no guarantee, but it principle, it is possible to install A2 that way. I cannot remember what CPU your Toshiba devices have, but as far as I know, A2 requires at least a Pentium. It should be possible to make it work on 486 CPU (not 486SX) without much effort. To make it work on older CPUs would be quite a lot of effort because we don't have a floating point emulation for Intel IA-32 (does NO have one?).