I gave my 2 TB Seagate external HDD to a fried who tried attaching it to her MacBook Air. She gave it back saying That nothing happened. So I read up a bit about NTFS and FAT32, etc, and didn't worry too much. Now when I attach my hdd to its own xp based laptop it doesn't recognize the hardware. lights and all blink during drive tests but it doesn't show up in hardware search.
Here is a checklist I hope you find it helpful.
1: Check the cables is there any damage?
2: Check your USB ports for damage.
3: Is the external drive making any loud noises? If noises then the harddisk could be damaged. Some Seagate enclosures have been known to have high failure rates.
4: Can it be used on another PC?
5: Is the disk showing up in Disk Manager? If it is the data may be retrievable. Download a boot disk and use data recovery tools.
6: Is the drive a Truecrypt repository? If yes check your password and remount.
On a separate note it might be worth asking the person you lent the drive these questions?
A: Did they drop the drive or did it get wet?
B: Did they attempt to format the drive for use on their MAC. If yes then use data recovery tools. DO NOT attempt to write to the disk as this will wipe or further corrupt the disk. Save data to another location.