Hi, thanks for your replies and my apologies for not getting back to you sooner. The USB external drive is an NTFS formatted Buffalo HD-PCU2 500 GB. (Although the "Properties" tab for some reason says the capacity is only 465 GB).
So, (in theory) I have about 65 GB of free space which I would like to format as Ext4 and use as a Linux partition. However, I cannot shrink the drive and partition it as these NTFS system files are sitting in two blocks at the end of the drive.
I should have also explained that my Windows OS is Vista SP2 (Home Premium). I'm intending to install Linux Mint as a dual boot in another partition on the C: drive.
The easiest thing, as has been said, would be to move the existing files and re-format and partition the drive. But I don't have another 500 GB drive available. Yes, I know I really should invest in more storage, but money's a bit tight at the moment.
Some time ago I did have the free version of PerfectDisk which probably would have moved these files. My trial expired and I don't want to pay for the full version.
I think I'm resigned to the fact that I will just have to find the cash for another HDD !