Hi bobby pin,
We're not a lot further forward. Tedious though it must seem, can you just give one-line answers to my previous questions please?
Is the EHD mains-powered or USB-powered? (useful because USB powered are more likely to get this problem especially if there are several attached)
Is it attached directly to the Mac or via a hub? (useful because unless the hub is powered, and even then, some EHDs can't get enough power)
The name of the EHD, Western Digital, LaCie, Seagate etc? (useful, some are more susceptible than others)
Does the "improperly ejected" message only appear after reawakening the Mac from sleep? (this is quite a common scenario. Mac goes to sleep & on reawakening doesn't recognise EHD, thinks it's been disconnected).
Have you tried a different cable from EHD to Mac (useful because cables can get damaged especially if regularly attached/unattached).
Thanks
Ian