How do know what Hd is in what bay in a mid 2010 Mac Pro ?

Go to About This Mac, then System Report, under Hardware click on SATA/SATA Express

Your drives will be listed, from bay 1 to bay 4, top to bottom

click on a drive to get more information on partition names, sizes etc

and confirmation of the bay number.

see screenshot below from my MacPro.

Depends on the OS version as to what Disk Utility can tell you. If you are using El Capitan, run About This Mac > System Report > Storage, click on the Mount Point heading above the drive list and this will lay out the drives in order of the bays they're in; top to bottom = left to right.

You should be able to tell by noting what the storage display says. Then shutdown and remove a data drive and reboot. Note the change the storage display. I would remove either the one closer to from or back, not one of the middle ones.IN my 2009 with a PCle startup SSD the startup SST is on tope and my two disks in the drive bays are listed from front of Mac to rear.

Yes I do know that, i have one myself

top to bottom refers to list of drives under system report, see screenshot.

the highlighted one is bay 1, below it is bay 2, etcetera, etcetera.

drive bay 1 is on the left inside the mac, you may have your sleds mixed up if you are

thinking that bay 1 is on the right.

bay refers to the point where the sled connects to the motherboard.

The numbers on the sleds are not computer-readable, they are only for your convenience. Bay one is above the fan at the front or left, depending on how you think of it.

I am well aware of the fact the the numbers are for our convenience.

the original poster had mentioned that he thought bay 1 was on the right hand side

so i surmised that he may have his sleds in non corresponding bays.

I pointed out that bay 1 is on the left.