I have a Mac Power PC G5. I have a Maxtor 750 MB external Firewire drive that does not show up on the desktop, so I am not able to access the drive.
you should turn it on. If that doesn't work, try Applications>Utilities>Disk Utility. If it shows up in the menu on the left, you just need to mount it - click the greyed out name and click "mount". If it's not there, unplug it, plug it back in and see if it shows up. If still nothing, it's probably dead.
Macs can read drives for all formats - FAT, FAT32, whatever...
If you just bought/received the drive and have no important documents on it, you should format it. To do that, open Disk Utility which is in the Applications/Utilities folder on your hard drive. Your external drive should show up in the list volumes on the left side of the window. Unless you have CDs/DVDs in your computer or an iPod plugged in, you will most likely see four icons, the top two referring to your computers built in hard drive (One being the model of your internal hard drive and the sub-level being "Macintosh HD") Your external hard drive will show up with a name including the size of it and the brand. Underneath that will be a sub-level. Select the drive, not the sub-level. Go over to the right side of the screen and select "Erase". Under volume format, select "Mac OS Extended (Journaled)" if you are only going to use the external drive with a Mac computer. If you are going to use it with a Windows computer too, select "MS-DOS file system". Under the name, name it what ever you want. Click the "Erase" button. Your drive should work now.
Your external hard drive is not formatted correctly to be read on macs. Backup the contents of the hard drive on a computer that can read the files, then format the drive as FAT or FAT32, or on a mac format it as MSDOS. It then should be readable and writeable on both windows and mac.
You should check if your drive is compatible with Macs. Some only come with drivers for Windows.