There was a long discussion about this in the private "Lounge" forum that unfortunately only level 6 & above users can see. (For level 6 & above users the topic is here.)
The gist of it is that quite a few WD Passport Studio 2 TB drives came from the factory preformatted for OS X with the same UUID. (A level 6 User confirmed that two he had been using regularly plus one fresh out of the box all had a UUID of 2CA31A4C-9B24-3F7D-A7A2-4B168E43FBBE. All were made in Thailand, but had different enough serial numbers to suggest they were made at different times.They were purchased from different sources.)
There was some speculation about how that happened, but clearly the error is Western Digital's, not something caused by OS X or users. You may want to contact WD customer support to see if they have any suggestions for how to fix this but the only thing I know of to correct it is to reformat the drives using Disk Utility, which will give each one a unique UUID.
This will erase the drives, so to save what is currently on them, I suggest that you connect them one at a time & clone their contents to another drive (obviously one with a different UUID!), reformat them, & then clone back their contents.
OK, i've been having a lot of problems lately with backup software (superduper! and time machine in particular), as well as itunes, where the apps seem to be confused about which external hard drive is which. this has had messy and in some cases disasterous results. after digging around a bit, i've discovered that an external hd (western digital 2t) connected directly to my mac, as well as three other hds (all western digital 2t drives) connected through a hub (anker usb 3.0 powered hub), have identical uuids.
anyone know why this happens? and what i can do to correct it?
As R C-R says I posted about this in the Level 6+ forum a few months ago.
I think all you can do for now is reformat prior to using them - I didn't notice until each one started being used as a Time Machine Backup when I'd only intended the first to be used for that.
You may wish to contact WD support - I nearly did at the time but didn't as you seemed to need to register to contact them and I was looking for a more straightforward contact method.