My external hard drive cannot read by my PC at all ,I connect it on Mac os x G4 and i could reformat it nicely,so why on PC.

check the compatibility of the external hard drive. It might not support Mac. Probabely you external hard is in NTFS format and you need to reformat/re-partition it for using it in Mac OS.

Connect the external hard drive to your mac
Open utilities folder from applications
click on disk utility
Highlight the hard disk you want to format. Careful not to choose Macintosh HD
Click Erase on the top
Select the format Macintosh OS Extended Journaled
Click OK. This will make your hard disk formatted to Mac OS
good luck

Disclaimer: the following process will reformat your external drive, and also erase all the data on it. There is no possible way to reformat the drive while retaining the data, as reformatting requires erasing.
Press cmd+space to open up spotlight (or just click the magnifying glass in the upper right corner) Type "disk utility" and the first result should be an app called "Disk Utility" and has the icon of a hard drive with a stethoscope. Hit enter.
Connect the hard drive to your computer, and you should see it pop up in the box on the left side of the window. You should actually see two things: a disk icon with the size of the drive and maybe the brand and, below it and slightly indented, a disk icon with the hard drive's name next to it. That's the one you want to click on. In the top middle of the window, you should see an "Erase" tab. Click on it. Pick "Mac OS Extended"* in the Format drop down Rename it if you want to in the Name text box
Click the "Erase" button
It will then reformat the drive to be compatible with Macs.
*Use Mac OS Extended if you plan on using it to store files. If you want to install Mac OS X on it, choose "Mac OS Extended (Journaled)". If you want to be able to access the files from a PC running Windows, you should choose "MS_DOS (FAT)", though I would not recommend doing that if you don't have to, because it is rather slow.

If you plugged this into a PC and formatted it you probably formatted it with NTFS or another Microsoft file format. This will not be recognized by a Mac. You will need to pop the hard drive back into your Mac and boot to the installation CD. From here you will need to go into disk utility and reformat the drive with a GUID Mac Extended format partition. If you open disk utility and the drive doesn't show up you probably have an issue with the drive controller on the logic board.

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Hi,

Check the file system of your hard drive. If your PC is recognizing it, check if its a FAT or NTFS. If it is NTFS then you have to reformat it using your Mac and choose FAT. Mac OS recognizes NTFS as a read only drive. If you plan using it with your PC and MAC you should format it using your mac and choose MS-DOS in disc Utility. You cannot format the drive using your PC Computer with a FAT file system because of the capacity. Only Mac can format it with a FAT file system.

You shouldn't need a special driver, but you do need to plug in the usb y cable to 2 usb ports. Normally 1 usb port doesn't supply enough power to make external USB drives go. That is why I prefer firewire, it supplies much more power at higher sustained rates. Once the drive is going just open the Disk Utility and format for your needs. If you need to share files with a PC, partition as MS-DOS (FAT). If you are sharing with a mac or not using the drive on other computers, format as extended with journaling.

Hi Sorry to hear you are having troubles,You will need a firewire 400 cableOK, first thing you must do is reboot the eMac in TARGET mode, this means it will be recognized as an external disk. To get the eMac in TARGET mode, you must be sure the eMac is turned OFF and has a functioning keyboard connected.Power on the eMac and very very rapidly press the ( T ) key on the keyboard, be sure the caps lock button is not pressed. If all goes well, you will see the firewire logo appear larger than life on the screen.( If you are unable to get into TARGET mode, it is highly probable that the mother board of the internal hard drive is damaged and preventing the eMac from entering TARGET mode, sorry, that's the end of the line..)Once you enter TARGET mode on the eMac, connect the firewire cable to your G4 PowerBook, you can do this either while it is running or when it is off and then turn it on.
You should see the volume of your eMac internal hard drive appear on the PowerBook desktop...sometimes you need to open Disk Utilities and repair or mount the drive manually.If this all works well, copy the required files directly to your PowerBook...After that, power off both machines, disconnect them and reboot the eMac using a CD or DVD to reformat and reinstall a new system.

Hello,
If you can see the drive in Applications --> utilities --> Disk Utility, then you will be able to reformat it in the Mac OS format and read it.
Good day, Damien

I would suspect a corrupted catalog file, these are the map that a comupter uses to find its way around the drive, they are also the first casualty when a crash corrupts something. Disc warrior will write a new catalog and fix everything. PCs do not see discs formatted for macs.

Well, you can plug it into your mac, and then open the applications folder on your mac(by opening the Macintosh HD icon first) and opening the utilities folder and selecting Disk Utility. Through disk utility, you can reformat any drive to Mac OS(preferably journaled.) so that you can use it with Time Machine, and the MAC OS.