Thanks pazos, nice to know I'm on the right track. I tried booting the first chunks as you suggested. uuu seems to hang with "Success 0 Failure 0".
After replugging the kobo, it appears in dmesg with
Code:
[[129961.122394] usb 1-1: new high-speed USB device number 48 using xhci_hcd
[129966.531420] usb 1-1: device descriptor read/64, error -110
[129982.403312] usb 1-1: device descriptor read/64, error -110
[129982.512361] usb usb1-port1: attempt power cycle
[129982.899300] usb 1-1: new high-speed USB device number 49 using xhci_hcd
[129987.922329] xhci_hcd 0000:00:14.0: Timeout while waiting for setup device command
[129993.552290] xhci_hcd 0000:00:14.0: Timeout while waiting for setup device command
[129993.760309] usb 1-1: device not accepting address 49, error -62
it keeps trying power cycles and eventually gives up with 'unable to enumerate USB device'
To get the kobo back into uuu mode I can just unplug it, hold the power for a few seconds and plug it back in.
Is there more debugging I could try from here, would a uart connection make this easier?