USB Harddrive GONE! Shut Down not working! HELP ASAP!

Very NEW TO MAC ! PLEASE HELP ASAP!!! I NEED TO SECOND HARDDRIVE!!


Yesterday it was all fine and today, I don't know. I have a SATA/300 Harddrive to placed in a Antec enclosure and hooked over UBS. I got this about 10 days again and it hooked up with easy. Last night I moved all my videos, iTunes music and photos the harddrive (it was showing on the deskstop). Last night I went to "shut down" under the apple icon and went to bed. When I got up I found my Mac didn't turn off. It was just showing the desktops, no icons and no dashboard. I had to hold the button down and reboot it. Then I found that the icon for the second drive was GONE!!! I changed USB ports and it didn't help. I thought maybe the enclosure was bad cause it didn't have the light on the front anymore. I took it back, got a new one, same kind, but still doesn't work! I looked at Disc U. and the second drive doesn't show on it either. Also when I go to "Shut down" it won't do anything, it closed my dock and icons. Both problems seemed to hit about the same time. Also I have noticed on HDD that when I first turn it on I can hear a low hum sound for about 5min then nothing. I did a repair under Disc Unities but no help either. Normal? I am very new to Mac but I hope someone can help! I lost all my files on the HD!!!!
Here some info, if you need more please let me know:
Machine Name: Power Mac G4
Machine Model: PowerMac3,6
CPU Type: PowerPC G4 (3.3)
Number Of CPUs: 2
CPU Speed: 1.25 GHz
L2 Cache (per CPU): 256 KB
L3 Cache (per CPU): 2 MB
Memory: 512 MB
Bus Speed: 167 MHz
Boot ROM Version: 4.4.8f2
System Version: Mac OS X 10.4.11 (8S165)
Kernel Version: Darwin 8.11.0
Boot Volume: Hard Drive

This is listed under USB Device Tree: USB High-Speed Bush
Not sure if it anything do with the HD

USB High-Speed Bus:

Host Controller Location: Expansion Slot
Host Controller Driver: AppleUSBEHCI
PCI Device ID: 0x00e0
PCI Revision ID: 0x0004
PCI Vendor ID: 0x1033
Bus Number: 0x54

USB2.0Hub:

Version: 1.00
Bus Power (mA): 500
Speed: Up to 480 Mb/sec
Product ID: 0x6254
Vendor ID: 0x058f

Mass Storage Device:

Version: 1.00
Bus Power (mA): 500
Speed: Up to 480 Mb/sec
Manufacturer: Generic
Product ID: 0x6366
Serial Number: 058F0O1111B1
Vendor ID: 0x058f