However, the speeds reported by Crystal Disk Mark seem normal to me:
I wasn't running anything else when I ran these tests. I have this problem under Windows and Linux. smartctl and badblocks detect no problems, although the latter runs very slowly. WD Dashboard doesn't find anything. I've run TRIM, removed and reinstalled the drive, etc.
Any suggestions as to whether the drive is defective, or whether something else might be the problem? As I mentioned, the problem appeared suddenly.