Sadly no. WiiU formatted HDD's are console specific.If your primary console dies (CFW), can you move your external hdd to a new console (also CFW) and have it work immediately?

If your Wii U is dying, you can perform a System Transfer from System Settings to a new console. Doing so will allow you to use any and all external storage devices used with the Source console (the dying one here) with the Target console after the transfer is complete.If your primary console dies (CFW), can you move your external hdd to a new console (also CFW) and have it work immediately?
If your Wii U is dying, you can perform a System Transfer from System Settings to a new console. Doing so will allow you to use any and all external storage devices used with the Source console (the dying one here) with the Target console after the transfer is complete.
Does a system transfer lock out the HDD out of the "dying" Wii U in this scenario?If your Wii U is dying, you can perform a System Transfer from System Settings to a new console. Doing so will allow you to use any and all external storage devices used with the Source console (the dying one here) with the Target console after the transfer is complete.

Yes. After the transfer is complete, the System Memory of the Source (dying) console will be formatted (so DO NOT perform a transfer if you still use CBHC for some reason) and thus will no longer be able to use the HDD. The Target console will be able to use the HDD immediately after the transfer.Does a system transfer lock out the HDD out of the "dying" Wii U in this scenario?