Will Backtrack be the only operating system on the external drive?
If not what else?
If so, what other files do you have? Do you have them on a separate partition? Are they Linux or windows?
but if I install it onto external drive can I stil access other files on the drive in windows
The "other files on the drive", are they windows or Linux? You'll likely have to install some third party software as I do not believe windows has the capacity to read Linux files natively.
More details on exactly what you have and what your intentions are would be helpful.
If Backtrack will be the only OS, you would just install Grub bootloader to the mbr of THAT drive.