How are you doing the installations? It sounds like you have it setup to stateless and so when the image server isn't accessible it boots off the local installation which is still ESXi5
I upgraded the server by booting the EXSI 6.0 iso burnt on a CD.
The install process find the 5.0 version on the SD card (the corresponding size is correct) and I upgrade it.
I think like firestartah.
When you reboot, your ESX boot from the SD. Disable boot from SD and try to update from CD.
I did update from the CD, after that I removed the CD and boot from SD.
I also make a try removing the hard drives during the upgrade to be sure it was correctly upgraded on the SD, no success.
I don't understand what firestartah is suggesting, I don't use stateless boot, only local installations & upgrades
what i'm stating is that the upgrade you are trying to do to the SD card isn't persisting and is rather writing the upgrade to memory. this could be due to your SD not having capcity to store the upgrade, your SD card not being on the HCL for 6 so it isn't able to persist the data to the card. Instead of upgrading can't you do a clean installation to see if it allows the formatting and then instalaltion of ESXi 6?
It sounds like a couple of things might be going on here. Like firestartah mentioned it might be due to an incompatible SD card however your previous version of ESXi is working.
For me another option is you might have a corrupted or failing SD card and after rebooting ESXi 6.0, the boot bank which contains 6.0 is not able to load, so reverts to the original boot bank which contains ESXi 5.0.
As suggested either blank (format) the SD card or try another one - you could even try some Linux tools to check the SD card for errors but this is probably going too far
Indeed my bootbank must be corrupted/failed, I tried to update my installation with "esxcli software profile update" but it fails, I have this kind of errors in esxupdate.log : No such file or directory: '/bootbank/boot.cfg'
I'll first do a clean install of the ESXI 6.0 instead of an update.
Excellent - let us know how it goes
The clean install did the trick. After severals reboot, it seems to keep booting an ESXI 6.0 version
thanks for your help