my macbook pro keeps ejecting my seagate external hard drive

FWIW - I've recently ran into this issue on Yosemite with a WD 4 Terabyte External drive. I was able to resolve by disabling Spotlight from indexing the external drive.

My WD drive has been a rockstar for about 2 years, and I've never had this issue. It only recently started when I purchased a new external drive and started migrating my iTunes media from 4 terabyte drive to the new one. Both are connected directly to a mac mini via usb (no hub). If I wasn't copying data from the drive, I didn't have issues. So I'm assuming that somehow copying data from 1 drive to the new one, and having Spotlight enabled, overtaxed the system somehow causing the random ejections.

FWIIW: as long as nothing changes on the disk that is activated for Spotlight, there is no indexing necessary, thus Spotlight does not use resources. As soon as you start copying files to the disk, Spotlight starts to index the disk for changes, and this can take a serious amount of resources, see it happen in Activity Monitor.

Personally I do not index any other drive than the Main Startup drive and the Drive where I have additional data for use (like photo-, music-, movie libraries, and other large projects). Backup drives I do not index.

Lex

Oh, well yes you must be trying to do some weird obscure off-shoot operation that isn't part of normal Apple computing. Oh, what? You're doing a Time Machine backup? Well, yes, Apple produces Time Machine, and technically supports it, but you must be doing something wrong. Your cable's bad. Your HDD is bad. So is everyone else's cable and HDD. Oh, what? It might be something wrong with Apple's hardware? First of all, that's impossible because we all know Apple doesn't make mistakes. Second, you're out of warranty so BUY ANOTHER MAC! 😉

This fix worked perfectly for me.