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The 5MB of unallocated space is a recommended configuration for the module to work properly, however it doesn’t mean that the module cannot be enabled if that space is not reserved. Depending on the data in the system, the usage and other factors, you might be able to enable the module as you have explained, but there is still the chance that in the future, the module fails because of this reason.
We always recommend to reserve this space from the OS installation process to make sure the module will work fine. You can keep using the module as you have been doing it so far, or if you prefer you can reserve this space at the end of the booting drive.
Additionally, and just for your reference, there are some error codes reported that are related to the 5MB required in the drive. Please check them here: https://www.intel.com/content/www/us/en/support/articles/000024113/memory-and-storage/intel-optane-memory.html?_ga=2.238670341.403999263.1540225813-1716436649.1536075640
I hope this information has clarified your concern. If there is anything else I can help you with, feel free to ask.
Have a nice day.