Hi, It is about iMac 2013 Late with original Mavericks installation, latest release .9.5. Problems as described below are encountered since approx 10 months. I'd like to ask you if these are known problems or rather my local issue. There are several hard drives available, non-ssd. And enclosures of different types, some of them or of the kind a adapter SATA to USB 3.0 others SATA to USB 2.0, yet others real enclosures. All drives are of same brand WD, but of different age. Some years old to quite new. Frequently while making a big file transfer (many MB or GB and of long duration) from or to or within such external they become disconnected in an unsolicited/unexpected fashion. If at such time they are logically inserted to os x the error message like "please eject correctly before disconnecting" pops up. The problems occur also if to use a non-OS X virtual machine hosted on this iMac for those data transfers. That means the transfer can start without problems and make a good progress for some duration but it aborts after some time due to issues described here. The experience that several hdd drives and several enclosures and OS X itself and the guest virtual machine are affected lets conclude me something with iMacs USB hardware or protocol stack might be wrong? How is your opinion?