Hi,
Making backups when powered down (shut down) makes the most reliable backups.
When the VM is suspended you can also make a backup, but you might have problems on restore if the virtual hardware is different at the time of restore. This has happened before when for example VMware added extra protections for CPU mitigations on spectre/meltdown. After you updated workstation with this mitigations, the CPU suddenly was slightly different and a suspended VM would not resume after restore. In that scenario you can still get your VM back to boot by discarding the VM state. But then you have a crash consistent backup.
You cannot make a backup while the VM is running, unless you use a product that is capable of doing so. At the moment I only know one such product and I'm the author (see my footer).
Hope this helps, let me know if you have more questions.
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Wil
