Hm. Okay, try not to boot WiiFlow or USB Loader GX until we have an idea of how to diagnose the issue, since unplugging it could damage the system. It's probably fine, but you know, it's not how it's supposed to be powered down.
Also, you don't have cIOS in 58 if it's just saying USB 2.0 -- it always says that in syscheck. And you said the slots 249, 250, 251, but not what the bases for them were. Here, I'll show a snippet of a syscheck I recently did -- if you saved your syscheck results to the SD card, you can find them in the file sysCheck.csv at the root of the SD card.
vIOS249[56] (rev 21011, Info: d2x-v11beta1-vWii): NAND Access, USB 2.0
vIOS250[57] (rev 21011, Info: d2x-v11beta1-vWii): NAND Access, USB 2.0
vIOS251[58] (rev 21011, Info: d2x-v11beta1-vWii): NAND Access, USB 2.0
Do you see the numbers in brackets next to 249, 250, and 251? Those are the bases used for the cIOS. The numbers 249-251 are just the slots to which the cIOS is installed, any base or version could be used though. So for my vIOS 249, the base would be 56. It's probably safe to assume that yours match mine, especially if you followed a guide, but it's better if we know for sure. That said if you just ran the syscheck but didn't save it, I'd understand not wanting to have to repeat it right now.
So, I have more questions. And don't jump to do them right away like they're suggestions, they're just questions. Have you tried loading either of these loaders without a USB drive plugged in? Have you tried a different SD card with fresh installs of everything, rather than copying any files over to it? Do you use UStealth to hide your HDD from the Wii U side so that you don't get the nagging message to format it? Have you checked your HDD on a computer to make sure it's functioning?
Actually that makes me realize another question I need to ask. What storage media do you use for these loaders? Is at an HDD, SSD, USB flash drive, SD card, or an SD card in a USB card reader? Certain storage media can be problematic. If it is anything plugged into a USB port, which USB port on your console is it plugged into? Is it plugged in using a Y cable, or in other words a splitter cable with 2 plugs on it, one for power and data while the other is just for power?
Again I would like to say, don't try loading those loaders again just yet, try answering some of my questions first if you can. Although checking your drive or SD card on a PC would be fine, just don't format it if it tells you that it's corrupted or anything.