HI all My Machine; Model Identifier: MacPro4,1 (Nehalem).Quad-Core Intel Xeon, 3,32 GHz,Number Of Processors: 1, Total Number Of Cores: 4, Memory: 8 GB I added 3 drives to my Mac in the 3 empty drivew slots available in my configuration. Once booting, one of them RANDOMLY does not mount. I have a Western DIgital Original with 10.6.8 System in it which came with the Machine. I never wanted nor attempted to initialize it. I purchased disk A (Western DIgital 1TB) disk B (Western DIgital 2TB) disk C (Western DIgital 2TB) All drives initiaiized with GUID, each of them with one partition. Not mounting RANDOMLY means at times (along with the HD with the 10.6.8 system) the Mac mounts disk Disk B and Disk C other times mounts disks A and disk C other times mounts disks A and disk B I not able to reproduce the error, as I tried to simply boot without running any application. Besides the additional drives only contain photos or films and a few text documents. There are no system folders in any of them. This suggest that the capacity of the disk is irrelevant. Each drive functions perfectly according to every available tests, Systm Profiler, Disk Warrior and othe Utilities They all display Make, Model, capacity and free space. To perform the test I remove one drive at the time so the machine has only 3 drives to mount.The drives mounted quickly and go through the Verify disk and Replace Directory (in DISK WORRIOR)without any problem. After swtiching off I move the drives arounf in the 4 slots to get same succesuful result. I repeated the test with every drive in every slot. I also instlled each drive in every of the four available slots in my MACPRO to make sure each slot hardware they worked Every time rebooting the Mac with 3 Hard Drives in any of the slots was succesful Every time I attempr rebootimg with 4 drives (one in every slot) the Mac only Mounts 3 drives.