I am in a similar position with an original C-4 which does most things and I purchased a C5 to handle additional stuff including mainly LAN devices. It might be better to switch them around and would be easy based on what you are saying regarding the dongles... however - didn't hubitat say the specs are the same?
I believe that they are the same apart from the internal radios on the C5.
IIRC the C7 is also similar specs to the C5 (might even be the same) apart from the internal Z-Wave radio is the 700 series.
Both the C5 and C7 use a 32-bit JVM whereas the C4 uses a 64-bit JVM. The 32-bit JVM seems to have a lower memory footprint which gives the C5 and C7 more headroom.
Anecdotally C4 users seem to be experiencing more issues with memory stability lately, possibly the more features that are added to the firmware releases the greater the burden is becoming. Up until several months ago I was running an older 2.2.4.x series firmware on my C4's with reasonable stability but since subsequently updating to 2.2.5 / 2.2.6 / 2.2.7 it does seem to be getting worse. They're both pretty static in terms of devices / apps and literally zero load according to the runtime stats, so I can only conclude the platform updates are slowly increasing memory usage.
There's a few threads floating around with people trying to track down C5's for an easier migration path as you can move your Nortek stick across.
Unfortunately that currently doesn't work on a C7, a couple of us have requested Hubitat to look into it so hopefully it's something they can implement. If not then C5 is the way to go ..... or suck it up and do a full migration to a C7.
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Thanks for the detailed reply.
I'll give that a go - switching over my c4 and c5 so that the bulk of the network is running through the C5.
There is another thread on here - this one:
Where people have been able to upgrade the firmware on their nortek stick, and then are able to do a proper native NVM backup of it. If Hubitat also allowed manually uploading an NVM backup it would be possible in theory to restore that backup to the Z-wave chip in the C7.. thereby enabling people to move from C4/C5 --> C7 without repairing all of. their devices... which can be impractical
Not sure if that will be possible for Z-Wave since it's an entirely new 700 series chip and I believe they're forced to use the new Z/IP gateway functionality so it likely depends on what that allows for backup / restore.
I believe that the ZigBee chip is the same though through all the versions, just internal on the C5 / C7. I don't think their current chip firmware supports backup / restore but could be possible in the future hopefully if there's a way to update it.
ZigBee coordinator migration is a bit of a black art from what I've seen since the EUI64 address is hardcoded into the coordinator chip. You'd need to move that to a new chip for a seamless migration. There's a few links on google that seem to suggest it might be possible in later chip firmwares e.g. GitHub - walthowd/husbzb-firmware: Nortek GoControl HUSBZB-1 / EM3581 Firmware update image
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I thought for zigbee there is no need to use the original stick when going from c4-->c5 and once you do the restore you can use the internal zigbee - is that not the case?
When you restore a backup from a C4 on a C5 no radio data (Z-Wave or ZigBee) is bought across.
For ZigBee you have to re-pair all devices - they do slot back in to their existing ID's though, so you don't have to redo any apps, rules, etc like you do with Z-Wave. But there's still some hassles around actually doing the re-pairing, especially configuration. I mentioned some of mine over at Anyone have a C5 they aren't using?
That's why it's currently quite appealing for C4 users to be able to move their Nortek stick over to a C5 as it means both Z-Wave & ZigBee devices from that stick carry across. It's literally as simple as moving the stick and restoring the backup as the OP mentioned.
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What if you are in the UK and have a separate stick for z-wave and for zigbee? if you bring over both sticks will it be just as easy?
I'm not sure if it works with multiple sticks, since the C5 only has a single micro USB port and needs an OTG cable to allow power and use of a port simultaneously.
Not sure if anybody has ever tried a USB hub on the OTG cable, if that did work then you could try both sticks .... not sure what would happen on the platform side though, whether it would detect both and do the right thing or get confused as it wasn't expecting to have to choose between 3 Z-Wave radios ....
I'm in the same boat. Does the c5 disable it's own internal radios automatically, when it detects the nortek stick?
Yes I it does. It seemed to work okay when moving from my C-4 using an OTG but ran into other issues like being unable to change the channel - not sure why though might have been something I did. In the end I just restored from a backup, removed all my Z-Wave device entries and did a discovery/pair on the Zigbee stuff. All my devices were found and worked with existing config. Wasn't too bad - I have 75 or so Zigbee and 60+ Z-Wave+ devices.
Note: I am using a C-5 for Zigbee and Cloud stuff and C-7 for Z-Wave+.
Ok so I noticed something a bit weird:
I never used the z-wave stick that came with the c5 because I was continuing to use the c4 for most stuff and c5 for additional processor intensive stuff.
Now that I am switching them around I took a look at the stick that came with the c5... and its a uzb7 - i.e. 700 series - so... does this mean if I copy across my z-wave backup to this using a computer I could just use that with the c7 snd get all the additional benefits straight away?
I am not experienced enough to give you some good advice for that. I think technically you could backup the Z-Wave table via some other sw (PC Controller maybe?) and restore to another one but don't know if you could with different models. Maybe someone else can chime in with this.
On a side note - UZB-7 came with a C-5? That's very cool thing although not a US configuration as we use the internal radios. We have to buy ours separately if we want one. Good news is the price is not bad at $20 USD.
Currently I don't believe the C7 will look at any external USB radios at all so I don't think that would work.
From what I've gleaned the ZigBee chip across C7, C5 and C4 (external Nortek) is the same, so technically I don't believe there's any reason why the Nortek wouldn't work in the C7 for ZigBee like it does in the C5. Obviously I don't know how Hubitat works internally but from a purely Linux OS point of view I would imagine it's just a case of pointing their software at the right port that's created when the stick is plugged in.
What stick are you currently using in the C4 for Z-Wave?
I still think that if you have a C5 to hand, the thing to test would be using an OTG USB hub such as https://www.amazon.co.uk/AuviPal-Adapter-Playstation-Classic-Raspberry-Black/dp/B083WML1XB and then plug in both the Nortek and separate Z-Wave stick.
I don't see any reason why that wouldn't physically work, as in the USB hub would be recognised and present both sticks at the OS level. It's just what the Hubitat software would do at the software level.
From other reports we know that the C5 will pick the Nortek for both ZigBee and Z-Wave over the internal radios, but I don't think anybody has ever tried to see what it would choose if presented with a third option i.e. a separate stick for Z-Wave.
Perhaps we don't need another data point but I converted my oldest C-3 to a C-5 a few minutes ago. It could not have gone smoother.
This particular Hub is ZWave only. Zigbee has always been disabled. I have had 3 development hubs for years and maybe 6 weeks ago I shut one down and "factory reset" it, put it back in it's original box, threw it on a shelf. Today, I powered it up and restored the C-3's backup. I had to re-register the C-5, but that took less than 30 seconds.
Then I powered down the C-3 and put the ZWave stick in the OTG of the C-5 and spent the next 15 mins hunting for a microUSB power supply, before remembering there was one in the original box under the cardboard (forehead slap) 
[I've been using POE for all my development hubs and thus it had been years since I looked at its PS. There's no POE in the room where the C-3 lived.]
In another topic, the C-3 and a C-4 I have, were showing high temps relative to the C-5's and C-7's. I had stuck a small fan an inch or two from the C-3 and it's temp dropped from 60-70C to 48C. Now the C-5 that replaced the C-3 has no fan aimed directly but is running at 31C, in line with the C-7's.
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During the C7 hardware beta I asked the question on whether adding a usb stick radio would work and was told by staff that it would not, as with the 700 chip additional sticks for different countries was not needed
Andy
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That's interesting .... so it will recognise an external (separate) Z-Wave stick correctly. And we know it recognises the Nortek stick as both ZigBee / Z-Wave.
I think @AdamV wants to know if (via an OTG USB hub) it will correctly use ZigBee off the Nortek stick and Z-Wave off a separate stick (rather than use Z-Wave off the Nortek stick).
Yeah that makes sense.
For ZigBee though the hope still stands .... that it might one day be possible to move the Nortek from an older hub to a C7, restore a backup and be back in business.
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Yea, it's a good puzzle. 
There would be 3 ZWave radios and two Zigbee. I think we already know that the platform picks an external over an internal, thus the external Zigbee would get the nod. But what about when there are two external ZWave? It will pick one, I'm sure of that, but which one and will it always pick that same one across reboots becomes the real puzzle for me.
There are C-5's shipped with the Nortek (ZWave & Zigbee) and C-5's shipped with just a ZWave stick (eg: zwave.me). Equally there are C-4s that were shipped with a Nortek for Zigbee and that regional ZWave stick (eg: zwave.me) and THAT is the case that suggests the platform will chose the "Not-Nortek" Zwave if it's available. Internal then External Nortek then External (other) is the implied detection order... and it sticks with the last found.
I tried this recently (just to test) and it didn't work for me. I can't rule out a bad OTG cable or even a bad stick , so I wouldn't take this as a definitive result, but my assumption is still tht it doesn't work.
Luckily, Zigbee is pretty easy to re-pair, even if it's not fun, so this doesn't concern me nearly as much as the work or redoing the same with Z-Wave would!
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Emboldened by my success with my C-3 I decided that today was an auspicious day to also move my (production) C-4 to a C-5. I have been using the C-5 as my only development hub and so I had to discard that use for now...
I did a Soft Reset on the C-5 and restored the C-4's backup. This particular C-4 has both Z-Wave and Zigbee via the Nortek stick. I then powered down both hubs and moved the stick. I use static DHCP so I had to adjust that, then powered up the C-5. And that was it.. both ZWave and Zigbee came back. (Zigbee took a couple minutes to stabilize (red dot in the upper right corner) but after that wait, it's all good.
Now my 4 Production hubs are: two C-5's and two C-7's. The C-3 is permanently shut. A C-4 is permanently shut and the other C-4 is now my development hub, having the backup from the C-5 restored on to it.
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That's what I was thinking ..... I have C4's with Nortek and external Z-Wave .... I don't use the external Z-Wave though as I use C7's for Z-Wave stuff. So the C4's are ZigBee only.
I really want to move from C4 for reasons already discussed many times, so currently waiting out in hope that a move to the C7 with the Nortek (for ZigBee) might work in the future. If not then I will probably follow others and try and track down some C5's.
Oh it definitely doesn't work on a C7 as I've tried it a few times now on my dev hubs, my guess is that the platform when running on the C7 is hard coded to look at the onboard chips only for both Z-Wave and ZigBee. So it would need Hubitat to put something in place like it seems they did for the C5, so it uses the external stick if it's there.
