When you copy files to an external drive, the system often reports completion before the data has physically finished writing. It keeps a short buffer in memory to make copying feel fast, then flushes it moments later.
Safe removal exists to flush that buffer. Pulling the cable before it completes can leave a file half written or, worse, damage the index that tells the drive where everything is, which is how an entire drive can appear empty after one bad unplug.
Drives that will not appear at all usually fail for the same reasons as any USB device: power, cable, or port. Larger mechanical drives are especially sensitive to underpowered ports and often need their own power supply.
The format matters too. A drive prepared on one operating system may be readable but not writable on another, which looks like a permissions fault rather than a formatting difference.