Synology DS207+ - Troubles with Volume - Disk 2 crashes

From time to time my Synology DS207+ suffers from a invalid RAID1 volume and telling me "volume 1 was degrade, please repair it". Well if I do so and start the repairing process my DS207+ works for the next 24h (up 3 days). After that period it tells me that disk 2 in volume 1 has crashed and that I should replace the disk.

Well the first time I really did replace the harddisk but after 3 months the same thing happend to this brand new harddisk. So I decided to take the harddisk out and put it into my PC. Starting my OS and deleting all partions on that disc. And well - oh wonder - this worked. To me this behaviour seems like a strange bug in the Synology firmware. Anyway to me this is the fastest work around to get my NAS up and running again.