Many times, a bus-powered external enclosure will lose power when the Mac sleeps, because the port will power down if there is not a continuous power draw (unlike, for example, a connected iDevice that is charging). When the drive loses power, from the Mac's perspective it's the same as unplugging it, and it force disconnects.
Here's what OWC (a reputable Mac hardware vendor) has to say about ejecting drives.
You could connect the drive enclosure via a powered hub, which should prevent it from powering down.
Another option is 3rd party software that automatically ejects peripherals when your Mac goes to sleep.
Popular Options to Automatically Eject Drives
- Jettison: Automatically unmounts all external drives before your Mac goes to sleep or locks, and automatically remounts them when you wake your Mac back up. It properly handles active Time Machine backups and network volumes so you never get data corruption or "disk was not ejected properly" errors.
- Ejectify: A lightweight utility (with source code available on GitHub) that runs in the background to automatically unmount external volumes when the Mac sleeps and remounts them upon waking.
- Auto Eject: A menu bar app from the Mac App Store that automatically ejects removable and USB-connected portable drives when you close your MacBook's lid, and can force them to remount on wake.
- EjectBar: Another App Store utility that lets you view and unmount all connected drives quickly, with options to schedule automatic periodic ejections.
