if the drive is formatted for windows and you need to use it on a mac it needs to be formatted in either Fat32, ExFat or NTFS (NTFS is readable on Mac OS but not writable without 3rd party software)
make sure it's plugged directly into the Mac, not a hub or keyboard
make sure it's getting power.
make sure the cable from the computer to the drive is working.
if this all appears to be in order and the drive is not seen make sure you have enable showing the drive on the desktop in mac
Finder>Preferences
again if all this is met and you still can't see the drive you may need to determine how the drive is formatted:
from Windows 7, go to Computer>"YOUR HARD DRIVE" right click and select PROPERTIES,
the type should say "Local Disk" and the File System will tell you how it's formatted.
from Mac OS click the spotlight icon (the magnifying glass in the upper right hand corner of the screen)
type "disk utility" without the quotes
and the drive should appear in the left hand column.
click the drive to highlight and the format should be shown.
if the device does not appear in the list :
go back to the spotlight icon
type "terminal" without the quotes
type "diskutil list" without the quotes
press return
if the drive is anything other than NTFS, FAT32 or ExFat the Mac will not read it (or write to it in the case of NTFS) without some software that is not part of OS X.
if all else fails you may be able to connect the drive to a windows computer on your network then share the drive to your mac. this will allow you to read and write to it without concern of what format the drive is in because the network will allow you to access the drive provided you set it up.
