In my testing when running Band-in-a-Box from an external USB HDD, open style from disk (command + U) presents the File Open dialog almost instantly, except the first time in a session when I first launch Band-in-a-Box where it takes about 5 seconds or so.

Try accessing the folder using the Terminal and see if it lists the files immediately. To do this, you can open a Terminal window and type
cd /Volumes/Band-in-a-Box\ Audiophile/Applications/Band-in-a-Box/Styles (edit the name of your hard drive it it's different)
and then:
ls -al

We would expect it to list files immediately. Either way, I'm not really sure what's going on there. There are some tools you can use to measure the read/write speed, and that may shed some light also.

Andrew
PG Music Inc.