You would use a Case() or If() calculation. It would look something like this:
Left ( Tape Number ; 3 ) = "sd-" ; return x ; return y )
How this read is, if the first three characters of Tape Number are sd- then 'do whatever you would to return x. If not, return y.
Use the Case and Left functions in your calculation:
Case ( Left ( Your_table::Tape number ; 3 ) = "sd-" ; return_URL_x ; return_URL_y )
Substitute return_URL_x and return_URL_y with the your URL concatenations.
The Case statement is basically saying that if the three characters starting from the left equal "sd-" then return_URL_x, otherwise return_URL_y as the default result.
LaRetta and Nick,
You are amazing. This worked perfectly.