I would want to stick with the MD600 but it seems like a roll of the dice. Hector if I may ask how many hours, or time in months or years did you have on your unit before it became unstable and was it only the drives and nothing else contained within the MD600 that was of concern?
Briefly in my case IIRC the failing item was a Seagate 3TB Drive. Naively I thought that the MD600 would report problems but as explained by VK above this is not so.
The Drive was over three years old and out of Seagate warranty.
Sod's law was also in play here as the failing drive was the one I made a copy of as a backup.
When I removed one drive from the MD600 I checked the number of albums utilising the C15, the results were different depending on which single disc was in the MD600.
The Disc with the lower number of albums showing was checked using a Seagate Disc checking application and could not be repaired or revived.
How long it had been in a partially failed state I do not know.
Before making a copy as a back up now, I carefully check that Album data on both MD600 discs (to the best of my limited ability) is identical.
Hope this makes things a little clearer.
Hector