Mechanical hard disk detects no bad sectors, but read/write speed is very slow, what happened?

There can be several reasons why a mechanical hard disk may be slowing down in read and write speeds. Here are some possible explanations:

Hard drive capacity is full: When a hard drive's capacity is nearly full, read and write speeds may slow down. This is because the hard disk needs to find free space available when performing read and write operations, and if the hard disk capacity is full, it takes more time to find available free clusters.

Hard drive fragmentation: Prolonged use and frequent file deletions and writes can lead to hard drive fragmentation, a state in which files are not stored contiguously on the hard drive. This can cause the drive to take longer to find and read the scattered fragments during read and write operations, resulting in slower read and write speeds.

Hard drive lifespan: Mechanical hard drives have a certain lifespan, and after a long period of time, a hard drive may experience degradation in performance, including slower read and write speeds. If a hard drive is nearing the end of its life, read and write speeds may be affected.

Hardware failure: Slower read and write speeds on a hard disk can also be caused by hardware failure. For example, a faulty motor in the hard disk drive, a problem with the cable connection, or a problem with the hard disk's read/write head may cause the hard disk's read/write speed to drop.

Solutions for this problem include:

Clean up the hard drive: delete unneeded files and programs, and organize the hard drive to reduce fragmentation to improve read and write speeds.

Checking the health of the hard disk: use a hard disk health checking tool, such as CrystalDiskInfo, to check the hard disk for health problems, such as bad sectors, SMART errors, etc.

Check hardware connections: Make sure the hard disk's data cable is firmly connected and not loose or damaged.

Consider replacing the drive: If the drive is nearing the end of its lifespan or the drive has a hardware failure, consider replacing it with a new one to fix the slow speed.