Is is possible to use the following PCI-E card with a HP DL180 server (specifically the 487507-371 model) - HP 4x Internal/4x External SAS PCI-E Adapter (416096-B2).
If not what are the options for attaching an external SAS tape drive?
Yep, that will work just fine assuming you've got an open PCI-E slot (the server has 2 full height, and 1 low-profile). The cable is going to depend on exactly what type of tape drive you're connecting it to. If you edit in the Tape part number I can find the cable for you too.