core won,t download to my external drive.
A stupid question, assuming you're talking about the core qt wallet, you did start it from the commandline like this:
"C:\Program Files (x86)\Bitcoin\bitcoin-qt.exe" -datadir=d:\BitcoinData
(given that you're using windows, and d:\ is you external drive)
If you're talking about something else (the bitcoind daemon, linux,...): please give a bit more info