Can I buy SAS drives on my SATA Controller?
What is the compatibility and restrictions about mixing these?
SAS offers backwards-compatibility with second-generation SATA drives. SATA 3 Gbit/s drives may be connected to SAS backplanes, but SAS drives may not be connected to SATA backplanes.
The SAS protocol supports tunneling SATA commands over it. Thus, a SAS controller can communicate with SATA drives or SAS drives. However, it doesn't go the other way. SATA controllers can only communicate with SATA devices.
Supporting mixed drive types is a vendor-by-vendor, device-by-device thing. Generally, this is not well supported and will cause both drives to run at the speed of the slowest drive. Support will likely improve in the future.
It depends on the controller, some HP ones support both but you'll have to check with your vendor specs.