is there an upper limit to the number of drives that can be connected?

one of my internals is 3TB, not 1TB, but the point is, the internals have been fine for months. The problem really started happening when I added a 2nd 8TB drive to my externals.

Thanks BDAqua,

None of the externals are bus powered. I use four powered docks, each with its own wall wart.

Some docks do have an 8tb limit, but even with each drive in a separate dock I have the issue.

Someone mentioned to me that there have been reports of problems when 8TB is exceeded on the internal SATA bus. Since I was just over that limit, I swapped the internal 3TB HD for a 500GB SSD, dropping the internal storage well below 8TB and restarted. This did not change the total amount of drives or storage--just moved things around. After a reboot, for just a few minutes I had all drives up, but alas, after about 10 minutes or so, one of the drives (the 3TB I had moved to a dock) was kicked off the bus.

No such luck. This is really weird.

Nope, a quick estimate is between 15-20 million files/folders total.

I thought also it might be the PCIe card, because I have had some reports of the card getting flaky when more than two of its 4 ports (2 USB3, 2 eSata) are used, but I've had an hour of stability taking all drives off of the built-in USB bus and putting everything else on either the card, or built-in FW. I'm not drawing any conclusions yet, because all of the temporary solutions that I thought would work turned out to be flaky and I don't expect this to last.

But if I get a single productive evening of a few hours without any more drives being kicked off, then I'll assume I've found some configuration that may work. Maybe I have too many external USB devices (I have a lot), and that's what's bothering my system.

Since my last post I haven't had any drives booted off the machine during normal operation. Everything is working since eliminating any hard drives being on the USB2 built-in bus. It isn't that I have anywhere close to the number of devices listed in the article as a limitation, but there are enough nested hubs and other devices (including a display link 17" monitor) that I think there's a bandwidth issue or something. Not sure about power although loss of a power adapter in back of my desk is possible. I haven't checked that. The only other thing I haven't checked is swapping docks, so it is possible that the dock I'm using on that bus is faulty in some way. Will test that scenario out eventually.

What nags at me a little though is that many times it was NOT the drive connected via USB2 that was being dismounted. Sometimes it was an internal SATA, or sometimes a dock attached to my PCIe card. So I have to surmise that the drives are being looked at in some kind of gestalt, and the OS makes it's own decisions about what to boot off when things get squirrelly even on another bus.

Thanks for the ideas!