no probs. Do I have to select any special options or just format as per normal?
You just need to select "Factory Default" - I know it's stupid, you'd think that means formatting it back to 4096 but no, it will format it back to 512.
But before you do that you can run this command from the windows dos prompt to see if your external drive is formatted as 4096 block:
fsutil fsinfo ntfsinfo f:
Where f: is the drive letter of your USB drive.
If it reports something like this:
NTFS Volume Serial Number :
Version : 3.1
Number Sectors : 0x000000000e8b4eff
Total Clusters : 0x000000000e8b4eff
Free Clusters : 0x000000000e8aa566
Total Reserved : 0x0000000000000000
Bytes Per Sector : 4096
Bytes Per Physical Sector :
Bytes Per Cluster : 4096
Bytes Per FileRecord Segment : 4096
Clusters Per FileRecord Segment : 1
Mft Valid Data Length : 0x0000000000100000
Mft Start Lcn : 0x00000000000c0000
Mft2 Start Lcn : 0x0000000000000002
Mft Zone Start : 0x00000000000c0000
Mft Zone End : 0x00000000000cc820
RM Identifier: 42643033-B240-11E0-9765-3C4A92E16BD6
(Emphasis' mine) then you need to reformat it back to 512k block. Otherwise the problem is else where.