Hi, welcome to TSF.
Sorry about the delay.
It sounds a bit odd. Some external drives get "registered" as local - probably because of the IDE > USB interface in the enclosure.
Even if the drive was badly fragmented you shouldn't see such a big difference between down- and up-load.
Do you have a "Safely remove" icon next to the clock when the drive is connected?
Try running a benchmark in PcWizard (link in my signature).