I upgraded an RB250 to 1.17 with the RB250 version thereof and it went swimmingly. I then proceeded to upgrade two RB260s and I bricked them both (the browser produced screenfuls of gibberish and then I lost the connection for good, though the switch keeps working as a dumb switch.) I can only speculate that I applied the wrong version for the software, however in the mikrotik downloads page the version I used is clearly labeled as the RB260 version. I downloaded it again and did a file compare and there is no evidence of corruption. On the other hand on the mikrotik downloads page I also see a 2.0 release that states "for new RB260GS(CSS106-5G-1S), new RB260GSP(CSS106-1G-4P-1S)" and another one that says "for CSS326-24G-2S+" How do I know if either of those 2.0 releases apply to my RB260s? (I don't see that type of CSS hardware version number anywhere on my boxes or on their PC board.) Even if my switches are that newer hardware version number I would think the regular 1.17 RB260 release can be applied to them with no problems (or do I HAVE to use the 2.0 version.) Any insight into the release to use will be appreciated.

My next step is to TFTP back down to 1.15 which I knew worked. I presume that downgrade can be done, right?

From your post I would say your RB260GS have some time already, isn't it? When did you buy them? Unless they're really recent, no need for the new 2.0 firmware. Check labels...

Have you tried factory resetting them?

Just upgraded a RB260GSP from 1.12 to 1.17 without issues.

How do I know if either of those 2.0 releases apply to my RB260s? (I don't see that type of CSS hardware version number anywhere on my boxes or on their PC board.)

Check the label on the bottom of your switch. The new ones are marked as CSS106-5G-1S (or CSS106-1G-4P-1S for RB260GSP) like this:

Even if my switches are that newer hardware version number I would think the regular 1.17 RB260 release can be applied to them with no problems (or do I HAVE to use the 2.0 version.) Any insight into the release to use will be appreciated.

My understanding is that these devices use completely different hardware, so 1.17 won't work on the newer ones (and 2.0 won't work on the older ones).

My next step is to TFTP back down to 1.15 which I knew worked. I presume that downgrade can be done, right?

I have an RB260GSP running 1.17 for almost a year now without problems.

Since you had 1.15 on them, it is an old hardware version. 1.17 should work.

Thanks for all the feedback. I tried the suggestions on one of the switches but had no luck. I tried factory reset, no change. I then tried tftp/bootp. I followed the instructions in https://wiki.mikrotik.com/wiki/SwOS#Reset_button exactly to prepare the router and couple the switch. The fast blinking light in the back of the switch never stopped blinking even after several minutes so I can only presume the upload was successful (some visual sign of completion would have been nice.) In any case, when I restarted the switch it was as dead (or rather as dumb) as before.

Both switches are 260GS. Judging by the label (thanks andriys) they are not the CSS series. Serial number on one of them in case it helps anybody is 416602C96D9F/428.

If it had happened on just one upgrade I would have said it's a hardware fluke but the fact it happened on both to me means there is a problem. I am certainly not upgrading my remaining 15 RB260s to 1.17. (The only small consolation is that since these 2 switches happened to be GS and not GSP it's a bit less of a writeoff...)

Email support directly explaining the issue...