@ulao Please check that your SD cards are fine. They should be formatted to FAT32 with 32 kb cluster size and the filesystem label (name) should not be wiiu. Also there's
https://3ds.hacks.guide/de_DE/h2testw-(windows).html to check the card for defects.
@weejew Sorry for this late reply but in case it is still an issue try to connect the power only end of your HDDs Y cable to a powerfull wall charger instead of the Wii U and see if that helps. NUSspli does very intense writes while downloading, causing some HDDs to create power demand spikes higher than under normal operation (like game loading which is mostly reading but not much writing).
//EDIT: Sorry
@ulao , missinterpreted what you wrote... So same to you: Try to connect the power only end of the HDD to a powerfull wall charger and see if that helps. No need to check your SD card for issues, sorry again.
//EDIT²: It could also be filesystem corruption on the HDD. I guess you don't want to hear this as it means to backup saves, reinstall all games and stuff but maybe try to reformat the HDD.
//EDIT³: Ofc. syslogs of this would be golden as that way we won't have to guess what's going on but would see it. This won't land at slc:/sys/logs/ through as it is no crash and I guess none of you knows how to grab the logs live from the running system?
The issue here is that the installation routine of the Wii U returns an error. Now ofc. I don't have access to documentation of the installation routine of the Wii U, so NUSspli just assumes the returned error codes means the USB drive is underpowered as this is most often the cause of it but I guess it's more of a general code for USB issues or something like that.