Get yourself a a usb y cable before you damage your wii u.

Will an external HDD that was designed to use an AC adapter work with a usb y cable instead?

I am also having the same type of problem and almost every time I turn the system on it warns me that I should not be disconnecting the hard drive while the system is turned on. Even though I haven't disconnected anything. Sometimes the HDD icon will appear in the bottom left corner of the GamePad display after the Wii U menu loading screen finishes, sometimes it doesn't. If I try to access the settings menu it will usually freeze the system. I am using a USB 2.0 Western Digitial Elements drive with 1.5 TB and its own power supply so I don't think the drive is the issue. Also when it does connect I can run downloaded games usually without a problem. I did have a very hard time getting the game downloads to work initially though and they failed due to disconnect errors multiple times. I also have a Deluxe system. I have not had any system beeping yet though during the freezes. I think there is an issue with how well the OS is detecting external drives and I'm hoping it is a software issue and not a hardware issue.