I have a Mac Mini 2010 with C2D Processor and 4GB RAM. I have upgraded it with MX300 275GB. But when I do a speed test for the SSD it gives a write-speed of about 110MB/S and a read-speed of about 130MB/S. It is supposed to give up to 500 MB/S on both. What is wrong?
Sounds like it's running at sata 1. You would need sata 3 to get 500MB/sec. Based on your computers age, odds are it is sata 2 and should be capable of 260MB/sec.
However, do you know what motherboard chipset the computer uses? Some old Macs use an Nvidia motherboard whose chipset is flawed and can't correctly negotiate sata 2 when faced with a sata 3 drive. Such a system will then revert to sata 1 unfortunately.