If it just suddenly started happening with no changes to the system I'd honestly be concerned about the drive starting to die, especially if it's old, since hard drives don't last forever. There's apps, like CrystalDiskInfo, that can test drives and read the S.M.A.R.T. status and current health reported by the drive, check for bad sectors, etc. In addition you can open a command prompt and use the wmic diskdrive get model, status command to read the S.M.A.R.T. status of all your drives. In a elevated command prompt (running as an administrator) you can also use chkdsk /f <drive letter>, e.g. chkdsk /f C: to scan the drive for issues.
Is it making any sounds that aren't normal or are strange? Like clicking noises? Is it freezing up and/or straight up disappearing in Windows sometimes? Is it connected internally via SATA or is it an external USB hard drive?