During the upgrade of ESXi in my NUC cluster I encountered an error that had something to with available space on the system disk(s). I have my ESXi installed on a USB device because I need to use both internal disks (2) for vSAN when I need it.

To resolve this issue I performed the following steps:

1. Download the ESXi Offline Bundle .ZIP
2. Logon to the ESXi host with SSH
3. Rename the /store folder: # mv /store /store.old
4. Remove the /store.old folder: # rm -rf store.old
5. Browse to the directory where the ESXi offline bundle is located:
   # cd /vmfs/volumes/SYNOLOGY-STORAGE/CIFS\ ISO\ library/VMware/vSphere\ 7.0/
6. Perform the update of ESXi via the esxcli:
   # esxcli software profile update -d /vmfs/volumes/SYNOLOGY-STORAGE/CIFS\ ISO\ library/VMware/vSphere\ 7.0/VMware-ESXi-7.0.0-15843807-depot.zip -p ESXi-7.0.0-15843807-standard

I hope this is useful for somebody. If there is anything to add or remark, please let me know!

sources: vmware.com