When i plug it in the computer instantly it recognized but when i detach it fro, the usb it just a blank screen

From the information I have, the unit will need to be repaired. No quick fix to repair for a blank display problem. Either the cable to the LCD screen became detached or the display is bad. The cable will be internal.

Consider NOT connecting your camera to your computer.
The best way to download pictures from your camera to your computer involves removing the memory card from the camera and plugging it into a card reader (either built-in to the computer or connected via USB or FireWire). This is likely to be faster than connecting the camera to the computer, and won't run down your camera's batteries.
Once the card is plugged in, it will appear to your computer as a removable drive. You can use the operating system's drag&drop facility to copy pictures from the card to the computer's hard drive, the same way you copy any other files. Or you can use any photo cataloging program such as Picasa.

If these drives are internally powered by the USB hub, two devices will not be recognised. ie: you can have as many pendrives as you like and they will be seen, because they draw little power from the hub, however, a larger external hard drive requires more power (some of these in fact have two usb connectors to power them). which is why your laptop will only recognise one drive at a time:

Solution: If you can get an externally powered usb drive, this should work.Another and possibly cheaper way would be to get an externally powered USB hub, which would solve the problem

Sometimes, portable drives need more power than a USB port provides. If this is a laptop, be sure it is plugged in to power and not running off battery. Also, if you have TWO usb plugs on the end of the wire, plug them BOTH in. This provides the extra power needed.

Just plug your USB HDD directly into your MBoard's USB port...

If stuck on initilizing then firmware went bad.
You need to go to RCA website download the latest firmware.
After you download the firmware.
plug in mp3 player to your usb device of pc.
After its plugged in install the firmware.
While its installing windows should find the palyer.
Leave it plugged in for about 1 hour to give the battery time to charge a little.

Did you try a different USB port?

If the drive has a power switch on the back, make sure it is off when you plug it into the laptop. Once it is plugged in, then power it on.

One other option is to plug it in before powering on your laptop and seeing if it recognizes it after boot-up.

It is always important to use the Safe Removal method when detaching USB drives from your computer...

try using a new battery and putting it on the computer to charge

if you plug in the camera before you down load the cd with the drivers you won;t be able to connect to the camera

I have to say Mac's I know nothing about. But the simple way would be to purchase a universal USB card reader- and use that.