I have done some research and found that unplugged hard disks slowly lose their magnetic states over time, therefore I need to refresh the data periodically.
Some answer in Superuser suggested I should periodically copy the entire data to a different disk. This will surely work, but it could be very cumbersome if you have a lot of data.
According to the following answer, a hard disk refreshes the data on its own when powered. This sounds much easier because I do not have to do anything but periodically power on the disk and keep it that way for a while.
A drive in use has the magnetic strength regularly refreshed, either by actual writing to disk or by the drive's own low level system, which periodically reads and rewrites sectors. (https://serverfault.com/questions/51851/does-an-unplugged-hard-drive-used-for-data-archival-deteriorate)
The questions are...
- Do all modern 3.5-inch hard disks (Seagate, Western Digital, Toshiba, etc) have this feature?
- Most operating systems spin down hard disks when nothing is read/written for a period of time. Should I disable this in order to keep refreshing going on? Or does a hard disk refresh itself even when it is spun down?
- Do they refresh only the sectors that have not been refreshed for a long time, or just from start to end sequentially?
- Is there any method to know if all sectors that need refreshing are refreshed? If not, generally speaking, how long should I keep the disk powered to refresh 1TB of data?