SSD and ram replacement of 2009 iMac stuck on loading bar

I think you’re hitting a known issue! Your system’s SATA port is only SATA II (3.0 Gb/s) While the drive you are trying to use is a fixed SATA III (6.0 Gb/s) drive which is too fast an I/O for your system.

This gets into the way the SATA standard was written. At the time the expectation was people would transfer their older drives to their new systems as HDD’s back then where quite expensive! Who knew the price of HDD’s would drop and the desire for deeper storage would also push us to larger drives.

In the meantime the SATA standard jumped from SATA II to SATA III doubling the throughput. Again, at the time the drives where either fixed or had a jumper to alter the I/O speed.

Later improvements in Ethernet speed detection rolled over to SATA! A group of drives where introduced which offered speed detection! So the drive would match its I/O speed to what the system needed (we call these drives Auto Sense).

Many people mistaken this technology as being normal its not! You need to review the spec sheet of the drive to see if the needed I/O speed is listed. In this case the Western Digital Blue SSD is a fixed speed SATA III (6.0 Gb/s) drive unlike this drive Samsung 860 EVO SSD note the Interface line lists your systems I/O speed.

As older SATA I & II systems are being retired or no longer being upgraded these Auto Sense drives are not as needed as in the past.

Many drive makers are dropping this tech as it effects the price of the drive. So to be more competitive trimming away no longer needed technology is being done!