Check Disk Management first
Press Win+X → Disk Management. The drive may appear here without a drive letter — right-click the volume and assign a letter.
Driver reset
Unplug the drive, open Device Manager, View → Show hidden devices. Look for any grayed-out disk entries and delete them. Reconnect the drive.
Try a powered USB hub
External HDDs draw more power than ports reliably supply. A powered hub ($15) fixes detection failures on USB 2.0 ports and some front-panel ports.
Disk needs initialization
A brand-new or reformatted drive shows as "Unknown / Uninitialized" in Disk Management — right-click the disk icon → Initialize → GPT.
Dead drive test
If the drive is not in Disk Management at all and the enclosure PCB lights up: the internal drive itself has failed. Remove it from the enclosure and connect directly via SATA to test further, or send for data recovery.