I have a new Dell with windows 7 and I have three external hard drives which I use on three other machines with xp pro one them. This machine will not open these drives and demands a format. They work perfect on my other machines
The only problem that I see that it could be is that the External Hard Drives are formatted with FAT or FAT 32 and the Windows 7 machine is formatted with NTFS. When you plug the external drive into the win7 machine it wants to format it with NTFS. You may need to format them with NTFS in order to use the drives with Windows 7.
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USB drives can be finicy so don't connect with a hub or use a cable longer than the one supplied and try a port on the back of PC. This sounds dumb but sometimes works with powered drives. Plug in voltage adapter into a wall outlet instead of a surge protector.