Ejecting disk after failed mounting
Mac OS X does have built-in read/write support for FAT32, and read support for NTFS, so it should be able to at least see any Windows-formatted volumes. That said, it's possible that Finder/Disk Utility cannot see it, but the system might still know that it's there and something is connected, so we might be able to safely unmount it via the command line.

To see if that's the case, open up Terminal, and type
diskutil list. This will output a list of all the volumes connected to the system, even if they're not mounted. They'll appear under a header showing what physical disk they're connected to. In the example below, diskutil is able to see a disk with a NTFS Bootcamp partition, as well as an HFS+ partition.


/dev/disk3  
   #:                       TYPE NAME                    SIZE       IDENTIFIER  
   0:      GUID_partition_scheme                        *500.1 GB   disk3  
   1:                        EFI                         209.7 MB   disk3s1  
   2:                  Apple_HFS mmotm                   409.0 GB   disk3s2  
   3:       Microsoft Basic Data BOOTCAMP                90.8 GB    disk3s3


To eject the pesky device, type in diskutil eject disk# substituting the # for the actual disk number (the part that appears after /dev/).

This has worked for me in similar situations.