Will An External Hard Drive Work On Both My Mac And PC ?

yes, it will work, but you need to make sure of the format of the external drive. Windows uses either FAT32 or NTFS. Mac OS X uses Mac OS Extended also known as HFS+... check this site, so that you will know more on what you should do when you are choosing one: http://www.myfirstmac.com/index.php/mac/articles/1-external-hard-drive-for-your-mac-and-pc

It depends. You need to make sure to that you format your External drive with a file system that both computers use otherwise one of your computers will not be able to read the external HDD

Yes, usually external hard drives are cross platform, which means they can be used on Macs or PCs.