I was asked in a private email about using a single SAS RAID HBA in a Mac Pro to support both a disk array and a LTO-5 tape drive or library.
Currently, the ATTO R6 family of SAS RAID HBAs is the only viable solution for a single slot. The other raid vendors have focused primarily on disk I/O performance meaning that tape I/O is simply not available, so you can't use Highpoint, Areca, or AOC with tape.
As to the ATTO, while their R-series HBAs support both disk and tape very well, the largest card for a scenario involving an external disk array and tape is the R680. Because that card has 2, 4 channel ports, you're limited to a 4Bay JBOD chassis or a more expensive unit with a SAS expander so that you can connect more than 4 drives to the 4 channel port. In the lower priced solutions (G-Tech, Sans Digital, etc.), the channels are simply passed through to the drives point-to-point. This means that you can only attach one drive per available channel. If you go with a more expensive external drive chassis that uses a SAS expander (Example Unit), you would use only one of the 4-channel ports to feed the disk array leaving the second available for tape.
Therefore, if you can live with 4 drives, then the G-Tech and Sans Digital solutions with the R680 are great (I use a Sans Digital on my system). However, if you require higher drive densities, you'll need to go with a SAS Expander-based chassis to support both disk and tape with the R680.