Firstly, I'm sure people will flag this as a repeat and post links to other stack exchange questions. However, after an evening of reading through everything on google I still cannot find a solution, although I'll be happy to be proved wrong on this. The problem is that the transfer speed between my mac and the usb hdd connected to my RPi is painfully slow, it takes about 10 mins to upload approx 100MB.
The details of my setup are as follows: the RPi is running Xbian only, I don't have anything else installed on the SD card. I added the samba add-on to Xbian and added the path /media/usb0 (which is the path to my external usb hdd) to the smb.conf file, gave it the name 'ExternalHD', and made it writeable and public. When I then drag and drop files to 'ExternalHD' from the finder window on my mac I experience very slow speeds. I have tried both wifi and ethernet connection to the RPi, no real change in speeds from that. My mac is connected via wifi, I haven't yet tested with ethernet at that end but intend to later. I also tried sftp from the terminal on my mac, again no real difference in speeds. My hdd is formatted to fat32.