Windows 7: usb disk formatting problem

I have a seagate 300mb external usb disk drive that was perviously connected to my old pc (windows xp). Due to a disk problem I had to format it for use with an apple macbook. I now no longer need it for the apple and want to use it with my new windows 7 machine. After connecting the disk, its recognised but I cant access it at all. I want to reformat it, but I dont have the option, in computer manager it doesnt even show up, yet it is in devices & printers. Im new to windows 7, so can anyone help me out ?

This might help - Disk - Clean and Clean All with Diskpart Command

the problem is the disk doesnt appear . in the list despite appearing in devices

Go to Disk Management (start > right click on Computer > Manage), then right click on the disk and assign a drive letter - or make a quick format with drive letter assignment. Make sure you choose NTFS.

the disk doesn't appear in the list of drives in disk management, but it does appear in the list of devices (if I unplug it, it then disappears from the list)

so I cant select it to format it.