I got this dell laptop from my employer and it had windows7 on it. I installed ubuntu on it and with windows side by side and it wont show ubuntu option during boot. I tried installing multiple times, ultimately I thought I would wipe out windows and install ubuntu. I did that and suddenly I got this message "Invalid partition table" I was like wtf happened.
I tried booting many times and same option, I thought I would press F12 and boot, and luckily i saw a weird option ubuntu under UEFI so I gave it a chance and it booted up. Now daily I had to press F12 to boot ubuntu and then after getting tired of this I remembered I had seen this UEFI somewhere in BIOS settings. I rebooted and pressed F2 and I saw under Boot sequence there was Legacy and UEFI. I selected UEFI and chose ubuntu as the option. Thats it no more invalid partition table on boot.