Thanks doughjohn, Sorry that it's taken me a while to get back to this, but it's been a busy week. The original HDD is in. I set the supervisor password. I'm not sure what you mean by set "Secure Boot", but I set it to require the supervisor password at startup and disabled "quiet start". I set boot sequence to DVD, HDD, EFI, removable, LAN. Unfortunately the results are the same. After I save the BIOS and exit, the system powers off briefly and then restarts with nothing on the screen. Curiously enough, the requirement for a startup password seems to be in effect as neither the HDD nor the DVD seemed to start seeking/reading as they have other times. No sign of the RAM and hardware check that you would normally expect with "quiet start"disabled. No sign of the ACER logo that should appear right at the beginning. Note - the checksum failure is appearing only because I've reset the BIOS by removing the battery. There's no reset jumper on this system that I've been able to find. I'm thinking this is a lost cause, but it would be really nice to at least know what I did that has caused this as I've cloned or rebuilt and changed HDDs on many systems before this, both laptops and desktops - mostly with Norton Ghost, but also several recently with Acronis since Ghost disappeared with the advent of Win 8. That said, I have seen some people having trouble cloning with Win 10 and Acronis (as opposed to backup and restore to a clean drive) and UEFI has been involved to some extent, but none of them appear to have lost the BIOS entirely.
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Thanks for your help. Symptoms are exactly the same now with the original 500 MB drive. I can't remember whether fast start was enabled or not. I remember looking at it and wondering which would be better, but not whether I changed it. If I reset the BIOS, I can get F12 (select boot) to work, but the only options it gives are "UEFI: Slimtype DVD" and "P3: Slimtype DVD". Although the BIOS config shows the hard drive (and it hasn't been changed since it was OK AFAIK), it doesn't show in the list of boot options when I get into the BIOS. Even if I select one of the DVD boot options, the system powers down before it tries the drive and then shows nothing on the screen even if the quick boot is disabled. Sequence I get after removing and replacing the battery is: .Boot selection if I've used F12 .POST saying the BIOS checksum was bad and default values have been loaded with F1 to continue or Del to enter BIOS config. .Whether I use F1 or Del and save any changes, the system shuts down briefly and reboots, appears to load from the DVD, but nothing appears on the screen and no use of Del or F12 on restart makes any difference unless I again remove the battery.
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I bought a secondhand L46720G and it ran beautifully, but had a 500 GB drive and I wanted a 1TB. I used Acronis to clone the drive (including the UEFI) and then installed that to boot, but it wouldn't recognise the disk. After several tries, I removed the battery to reset the BIOS and since then nothing on the screen at all. Thinking I'd done something that stuffed the mobo, I tried a second one with the same config. The original booted fine with the monitor, but once I put the TB drive in, it also refused to boot form that or a CD/DVD for Acronis or Win 10 repair disk. That one will now show the BIOS screen after the battery is removed to force a reset, but then it saves the BIOS and reboots when I get nothing - power is on and the disk is spinning (or the DVD is seeking, if there's a disk in the drive), but nothing shows on the screen. It seems that I've done something to stuff up the UEFI, but I don't know what - or if it's recoverable.
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