I tried this last night and it chose to unpack the hotfixes and the like onto the same partition on the external hard disk. Once finished unpacking, it simply stopped saying it could not continue but without giving any reasons. Even the log was a bit useless - the corresponding entry read, ‘The patch installation could not proceed due to unexpected errors’. But the fix is simple. Copy the service pack to your internal hard drive, switch off the external disk and install it from C:. Works fine then.