Your iMac has an older ATA I/O interface (ATA-I 1.5Gb/s) so any newer HD you put in is likely to be SATA-III 6.0Gb's which is just to fast for your system. The good news is many of these HD's have a jumper which allows one to slow the I/O speed down to be compatible with older systems.
Review the spec sheets of the HD's you have on the manufactures web site to see which of the drives you have can be slowed down via a jumper. That should solve your problem