The hard drive carrier marked SAS/SATu is a universal carrier that will hold SAS or SATA drives. You should see two sets of mounting holes on the hard drive carrier. The interposer board Is simply there to bridge the gap between the SATA drive and the backplane. The interposer board does not add any features. 
hello, for the first time in years we had a look at the drives mounted in our poweredge 2950. i noted that the two disks  0,1 are 80 gb (hot plug) 2,3,4 are 250 gb sata2 universal (usata2). i also noticed that these 3 disks have an adaptor plate mounted in the back. but i am puzzled as to 1. what is the practical difference between the two plate types. is it only that the 250 gb plates are not "hot" swappable ? 2. all disks themselves are marked as sata disks. what are the adaptor plates for ? 1000 thanks for any insights greetings ingo
universal sata2 vs sata2 hot plug