Recently, I discovered that my hard drive automatically ejects every time Time Machine starts a backup—yet the drive functions normally during everyday read/write operations. After some thought, I concluded this is likely a power supply issue.
Time Machine’s mechanism differs from ordinary file copying: it triggers high-frequency, small-fragment synchronization reads and writes, making it extremely sensitive to I/O latency and power supply stability. If your drive works fine for regular read/write operations but fails during Time Machine backups, insufficient power supply is usually the culprit (the most common cause).
During indexing and the initial backup phase, an SSD’s instantaneous power consumption spikes significantly. When connecting a high-performance NVMe drive enclosure to a MacBook’s USB port, voltage instability may occur—especially if using a low-quality cable or connecting through a non-powered USB hub—causing the controller chip to trigger protective disconnection (“drop-off”).
You can try replacing the data cable with a higher-quality one, or switch to a self-powered drive enclosure or a self-powered docking station.