I'm beginning to think this is now typical of all USB drives, which come pre-formatted for Mac's (with HFS) instead of Windows (with NTFS).
Anyway, since "delete volume" is the only thing you can do, I suppose you could select that. I have never done that before, but I suspect that 485GB partition will be deleted, and become "unallocated".
Given what this probably implies, I'd imagine the other two partitions on the drive should also be disappeared the same way, making the entire drive "unallocated".
Then, you should be able to create new partition(s) from this 100% unallocated space. Personally I'd recommend "logical" partitions rather than "primary", but it's not critical as long as you only want 1-4 partitions on the drive.
You perhaps will need to FORMAT to NTFS after allocating the partition, given what you've run into so far.
(1) Download and install [free] Partition Wizard Home Edition v5.2
(2) Also download the ISO image
and burn the standalone boot CD version of Partition Wizard, which can be used conveniently when the functions you're performing cannot be completed while booted to the OS, but must be run outside of the OS.
(3) Run Partition Wizard while under Windows 7 because work on this external drive (Disk1 for DISKMGMT, Disk 2 for Partition Wizard) can be run this way without any limitations.
Maximize the window to fill the screen, and spread the columns where information is shown, so that you can see all text in all columns. It's just much more readable that way.
(4) Select that external drive which will be titled "Disk 2" in the presentation, by clicking on the "Disk 2" title itself, not on any of the detail lines underneath it.
(5) When you select the drive itself (i.e. "Disk 2") on the left side of the screen you will see an available item under "Operations" named "delete all partitions". Select that "delete all partitions" and reply "yes" to the confirmation asking you if you want to delete all partitions.
This will add the "delete all partitions" operation to the queued list of operations now shown at the bottom-left corner of the screen. And the presentation for the contents of Disk 2" will now be modified to appear as if the entire 500GB (i.e. maybe 486GB after formatting) space is now unallocated and fully available for partitioning.
(6) Push the APPLY button at the bottom left of the screen, to perform this one pending operation to "delete all partitions" on the external drive "Disk 2". This will perform and complete it, and the list of queued operations will be emptied.
(7) Select the "unallocated" space now appearing under the "Disk 2" title. Now, on the left side of the screen in the "Operations" section you will see the "create partition" item.
Select this "create partition" item, to bring up the "Create New Partition" window. The options on the window should be completely intuitive, and you can either allocate it completely as one partition or you can create two or more smaller partitions in the 486GB available space. BE SURE TO SPECIFY NTFS FILE TYPE.
You use sliders and/or entered values to specify the size and location of the partition you're creating, and you can also install a label for the partition.
My suggestion is that you create the one or more partitions on this second drive as "logical" partitions instead of "primary partitions. Functionally, they're essentially identical, but with "logical" partitions you don't have the "maximum of four primary partitions on a basic hard drive" to contend with. You have no limit on the number of "logical" partitions. Obviously if you only want to have one partition on this drive it doesn't really matter whether you use "logical" or "primary".
(8) When you push the OK button, the "create partition" operation will be queued in the list at the bottom-left corner of the screen.
You can now repeat the "create partition" process a second time if you want to build more than one partition into that 500GB drive. Pushing OK will add this second partition's create to the queued pending operation list.
Repeat again as required.
(9) When you're done, push the APPLY button in the lower-left, and the one or more partitions you want to create on the external hard drive will be created.
(10) If for some reason the file system NTFS doesn't appear on the partitions, you should select the partitions and then choose the "FORMAT PARTITION" on the left side (or, right-click on the partition and select "format partition" from the popup menu).
Again, we want to get these partitions formatted as NTFS.
(11) You will get default drive letters assigned by this process. You can use Partition Wizard or Windows 7's DISKMGMT.MSC to change the drive letters to be whatever you want.
Again, if you use Partition Wizard to change the drive letters, you just select the partition from the list in the upper part of the screen or the graphic presentation in the lower part of the screen, and then select the "change drive letter" operation in the "Operations" group shown on the left side of the screen.
As before, push OK after you're finished setting things up, and you can repeat if you want to change another drive letter. All of these operations will again be queued into the pending operations list at the bottom-left corner.
Again, push APPLY, and the drive letter changes will occur.
Partition Wizard. Download it, install it, use it. You won't be sorry.