So I've just been through this with support and here's the steps taken. I'll caveat this with I dropped in to chat with support and was helped super quick and my hand was held all the way - so would recommened that as first option.
1. Switch off the unit (hold power button down)
2. Eject each drive a few inches when powered off (but leave in the unit)
3. Restart the unit, it should show a blue light on the power button (I had to restart twice to see this)
4. Check Promise device can be seen. For me this was Apple in Menu -> About This Mac - System Report, then find Thunderbolt in menue bar and my Promise device is listed as connected.
5. In System Report still, find the Software, Extensions menu item. Look up PromiseSTEX extension. Look at location option and copy this. For me it was /Library/Extensions
6. Open Finder and go to the folder from item 5 above.
7. Find PromiseSTEX.kext file and send to trash
8. Open Terminal window and issue command: promiseutil
This should open up a command line with cliib> and a load of info listing the Pegasus device
9. The pacakage R_MacDrv_V6.2.9.pkg was then downloaded and run, to install the driver
10. Reboot system
11. Asked to update to 4.04.0000.42 - said yes and allowed to install
12. Software asked to restart, so agreed. PromiseUtility restarted and window was showing (I had multiple monitors and disconnected to have only 1 to ensure utility did not get confused)
13. Insert each disc ONE AT A TIME and monitor on the Physical Drive option in PromiseUtil window
14. When complete, the drives should mount as normal and be accessible
Hope that helps.