How many external drives can I connect at a time to a new iMac?

[stanw]

1. How many external hard drives can I connect at a time to a new iMac?
2. Will having all these hard drives connected at a time slow down the iMac even if they are not being used?


Thanks.

 

[jaxhunter]

A lot. And if all those drives are part of your Spotlight index, yes you could see a critical slowdown.

Here's the numbers:

4 USB ports, 127 devices per root hub: 508 Devices.
2 Thunderbolt ports, 6 devices daisy chained per root hub: 12 devices
Or, Theoretically, based on current Thunderbolt docks, 3 USB 3 root hubs per Thunderbolt dock, 6 Thunderbolt Docks, 18 USB Ports as root hubs, 127 devices per USB root hub: 2286 Devices.
Maximum number of USB Devices (theoretically): 2794.

That's just based on the most rudimentary of numbers. Using a newtork, that number skyrockets even further. Mac OS X has no drive limits. There are limits per volume and per drive, but no drive limits.

So if you used your imagination and every port on your iMac you could theoretically connect every drive on the planet to your iMac.

In practical terms, you probably want to go ahead and limit the amount of drives connected at one time to however many you can sort through and keep track of. My limit is about 10.

 

[stanw]

Hi jaxhunter,

1. What did you mean by "if all those drives are part of your Spotlight index, yes you could see a critical slowdown." Do you mean if the drives are setup to be searchable via Spotlight? How is that even done? I just assumed that any drive connected was searchable by Spotlight.

2. A more realistic scenario for me might be if I keep 2 bare bone harddrives connected via TB in 2 separate docks and maybe 2 USB3 bare bone hard drives in 2 other separate docks, and I might eject drives all the time and swap them out. Are there any issues with keeping these connected and swapping them out all the time? Speed of the computer or transfer rates or other issues?

thanks.

 

[Bear]

...
4 USB ports, 127 devices per root hub: 508 Devices.
2 Thunderbolt ports, 6 devices daisy chained per root hub: 12 devices
Or, Theoretically, based on current Thunderbolt docks, 3 USB 3 root hubs per Thunderbolt dock, 6 Thunderbolt Docks, 18 USB Ports as root hubs, 127 devices per USB root hub: 2286 Devices.
Maximum number of USB Devices (theoretically): 2794.

I think your numbers are off a bit, some high and some low.

• 4 USB Ports on an iMac, but I think it's 2 root hubs with 2 ports each.
• Thunderbolt dock: Up to 4 USB ports, possibly firewire and estata ports. Unknown number of root hubs for the USB ports.
• Also it would be 12 Thunderbolt docks since there are 2 thunderbolt ports.

1. What did you mean by "if all those drives are part of your Spotlight index, yes you could see a critical slowdown." Do you mean if the drives are setup to be searchable via Spotlight? How is that even done? I just assumed that any drive connected was searchable by Spotlight.

Each drive has it's own spotlight database. Spotlight can be disable on a per drive basis (also can just exclude folders.) The more spotlight databases you search, the longer it can take to get get a result. However, for a handful of drives, this is probably not an issue.

 

[jaxhunter]

Yeah those numbers were just quickly off the top of my head. Mostly because the question "how many drives can be connected to a Mac" is a very large number.


As far as keeping 4 drives connected and swapping them in and out of their respective docks that should be fine if you properly eject the drives each time.