[X-Newbies] Crashed USB HK isn't recognized by Mac

[Aldo]

I was copying stuff to an external USB harddisk when my Mac crashed. 
When I restarted, Disk Utility immediately popped up, saying that the 
external harddisk had problems. I tried to erase the disk, but then Disk 
Utility said it can't due to i/o error. I tried to erase the HD again, 
but Disk Utility then hanged. I had to force quit it.

However, since then, I can't even get my Mac to recognize the external 
HD. It won't appear on the desktop, and on Disk Utility either. System 
Profiler can recognize it, but not Disk Utility. According to System 
Profiler, the HD is a "Journaled HFS+" file system. But how come Disk 
Utility and the Finder can't read it?

I tried plugging and unplugging, I tried to connect it to another USB 
port, I tried restarting with the HD connected and running. All to no 
avail.

How can I then erase/fix the HD?

Does this mean I have to get another disk enclosure for the HD? Or does 
this mean that I need a brand new HD?

Also, can a Mac crash really screw up the physical USB circuits? Or is 
it just a screwed s/w in the bowels of the OS somewhere?

My Mac is Mac Mini sys10.4.1. The ext HD is a generic Myson Century, 
inc. HD inside a generic USB2.0 enclosure.

Thanks in advance