MacBook Pro External Drive Ejecting Itself

[aliengin]

I have this weird thing happening...
I have a new late 17 inch MacBook Pro and 2 external 6TB WD Thunderbolt drives.
I purchased one TB drive with the Mac and been using it for 3 months without any problem. Now I just wanted a second one to use as a backup.
As you know these devices can be daisy chained. When I do that and start copying files from old TB drive to the new TB drive, the new TB drive ejects itself after a while. this could be 15 minutes or an hour later.
I thought the drive was bad and replaced it but it wasn't. I swapped the cables, change connection setup, used SuperDuper instead of copy and paste STILL THE SAME.
I noticed the drive ejects itself only when writing a lot of files. If I copyfolders one by one it seems working fine but once I highlight and add all 4TB data it ejects after 15 minutes or so.
I checked its fine, my software is upto date, contacted Apple support and Western Digital support no luck!

ONE minor thing, my Apple came with 4GB ram and I replaced it with 8GB. I checked the ram with Rember and seems they are OK no issues there. Could it be a bad ram?
I even reinstalled Mac OS last night and same problem.

[Hard Drive Disk]

It's the drives.

[aliengin]

It is? What could it be? Its not defective, I replaced it. Its working fine and ONLY ejects while copying a lot of files like 4TB of RAW files.

[crn3371]

Maybe it's overheating. Try transferring in smaller increments.